Method for producing a polymer electrolyte membrane

ABSTRACT

A method for producing a polymer electrolyte membrane having a strong acid group or a superstrong acid group is provided. The method includes casting on a support a polymer electrolyte solution containing from 0.0005 to 2 parts by weight of a phosphate ester represented by formula (11) and/or a salt between an amine represented by formula (12) and a phosphate ester represented by formula (11), with respect to 100 parts by weight of a polymer electrolyte; heating the solution until a solvent of the solution is evaporated to form a self-supporting membrane; and removing the self-supporting membrane from the support.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of co-pending application Ser. No.11/885,598 filed on Sep. 4, 2007, which is a National Stage ofPCT/JP2006/304067 filed on Mar. 3, 2006, which claims foreign priorityto Japanese Application Serial Nos. 2005-060778, 2005,243728,2005-243729, 2005-309331 and 2006-051530 filed on Mar. 4, 2005, Aug. 25,2005, Aug. 25, 2005, Oct. 25, 2005 and Feb. 28, 2006, respectively. Theentire content of each of these applications is hereby expresslyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a polymer electrolyte composed of anaromatic block copolymer having an improved proton conductivity, anelectrolyte membrane thereof, and a fuel cell using them.

The polymer electrolyte of the present invention and electrolytemembrane thereof have an excellent proton conductivity, so that they aresuitably used for fuel cells.

Further, the present invention relates to a polymer electrolytecomposition used for fuel cells, ion exchange membranes and others, andthe use thereof, particularly to a hydrocarbon polymer electrolytecomposition having sulfonic acid groups. In addition, the presentinvention relates to a polymer electrolyte membrane that is composed ofthe polymer electrolyte composition used for fuel cells, ion exchangemembranes and others.

Still further, the present invention relates to a fuel cell that employsthe above-mentioned polymer electrolyte composition and polymerelectrolyte membrane.

Still further, the present invention relates to a production method of asulfonated polyarylether block copolymer, and a block copolymer that issynthesized by the production method.

A polymer electrolyte and an electrolyte membrane thereof that arecomposed of the sulfonated polyarylether block copolymer synthesized bya production method according to the present invention has an excellentproton conductivity, so that they are suitably used for fuel cells.

Still further, the present invention relates to a polymer electrolytemembrane used for fuel cells, ion exchange membranes and others, and aproduction method of the polymer electrolyte membrane, particularly to aproduction method of a hydrocarbon polymer electrolyte membrane havingstrong acid groups or superstrong acid groups.

BACKGROUND ART

In view of recent environmental problems, there has been growingexpectation for fuel cells, particularly for polymer electrolyte fuelcells using a proton conductive polymer electrolyte membrane because thefuel cells can be operated at low temperatures and have a possibility ofreduction in size and weight. As the polymer electrolyte used for thepolymer electrolyte fuel cells, there is known a fluoro-polymer havingsuperstrong acid groups that is represented by, for example, “Nafion”(Nafion is a registered trademark of DuPont, hereinafter the same).However, the fluoro-polymer having superstrong acid groups is extremelyexpensive since it is a fluoro-polymer, and also the fluoro-polymer hasa relatively low heat resistance, a high alcohol permeability, and adisadvantage that environmental consideration is necessary onpreparation and disposing of the fluoro-polymer.

In order to address the disadvantages that the fluoro-polymer havingsuperstrong acid groups is expensive, and has relatively lowheat-resistance and high alcohol permeability, a number of proposalshave been already submitted for a polymer electrolyte membrane based ona less expensive non-fluoro polymer. In particular, in view ofdurability, heat resistance, and cost of the polymer electrolytemembrane, a hydrocarbon aromatic polymer electrolyte is desirable. Asthe polymer electrolyte membrane, there are disclosed, for example, inJapanese Patent Application Laid-Open No. H11-502249 (Patent Document1), Japanese Patent Application Laid-Open No. 2002-524631 (PatentDocument 2) and others, a sulfonated polyetherketone polymer electrolytemembrane; in Japanese Patent Application Laid-Open No. 2000-510511(Patent Document 3), Japanese Patent Laid-Open Publication No.2003-64181 (Patent Document 4), Japanese Patent Laid-Open PublicationNo. 2003-68326 (Patent Document 5) and others, a sulfonated polyimidepolymer electrolyte membrane; in U.S. Pat. No. 5,403,675 (PatentDocument 6) and others, a polyarylene polymer electrolyte membrane; andin Japanese Patent Laid-Open Publication No. H10-21943 (Patent Document7), Japanese Patent Laid-Open Publication No. H10-45913 (Patent Document8), Japanese Patent Laid-Open Publication No. H11-116679 (PatentDocument 9) and others, a sulfonated polyethersulfone polymerelectrolyte membrane. Further, Japanese Patent Laid-Open Publication No.H11-67224 (Patent Document 10) and others disclose a membrane/electrodeassembly using a sulfonated polyethersulfone polymer electrolytemembrane.

The improvement of properties on water-absorption has been requested,and a block copolymer has been proposed as the means for solving theproblem. For example, there are disclosed, in Japanese Patent Laid-OpenPublication No. 2002-60687 (Patent Document 11), Japanese PatentLaid-Open Publication No. 2004-190002 (Patent Document 12), JapanesePatent Laid-Open Publication No. 2004-190003 (Patent Document 13),Japanese Patent Laid-Open Publication No. 2004-346305 (Patent Document14) and others, polyether; and in Japanese Patent Laid-Open PublicationNo. 2002-358978 (Patent Document 15), Japanese Patent Laid-OpenPublication No. 2003-234014 (Patent Document 16), Japanese PatentApplication Laid-Open No. 2003-511510 (Patent Document 17) and others,polyimide. Further, Japanese Patent Laid-Open Publication No. 2003-31232(Patent Document 18) discloses a polyether sulfone block copolymer thathas an improved temperature dependence of proton conductivity. However,there is not any description on the relationship between the ionexchange capacity and proton conductivity of the hydrophilic segment ofthe block copolymers in these Patent Documents. Still further, JapanesePatent Laid-Open Publication No. 2005-126684 (Patent Document 19) andJapanese Patent Laid-Open Publication No. 2005-139432 (Patent Document20) also disclose a sulfonated polyether block copolymer, but there arenot any descriptions about the proton conductivity under low-humidityconditions, also there are not any specific descriptions about themembrane properties on water-absorption.

As the above-mentioned block copolymers, particularly a block copolymerexemplified in Japanese Patent Laid-Open Publication No. 2003-31232(Patent Document 18) and others is desirable from the viewpoint ofheat-resistance, durability and cost, since the block copolymer has apolyarylether sulfone type or a polyaryletherketone type on bothsegments, having no specific linking group that connects both segments,and is synthesized not through a reaction using a metal complex such ascoupling reaction. In particular, it is desirable because of low costthat the block copolymer is prepared as follows: a prepolymer serving asa hydrophilic segment is synthesized using a dichloride compound havingsulfonic acid groups as a source material; and then the prepolymer and ahydrophobic segment prepolymer that is synthesized separately arereacted through ether-exchange reaction.

Japanese Patent Laid-Open Publication No. 2003-206354 (Patent Document21) discloses the production method of a polyarylethersulfone blockcopolymer using ether-exchange reaction. However, this documentdescribes about a hydrophilic segment prepolymer having no sulfonic acidgroups. There are not any specific descriptions about the method ofsynthesizing the block copolymer through ether-exchange reaction using ahydrophilic segment prepolymer that has sulfonic acid groups

As the dihalide compound having sulfonic acid groups that is used forthe synthesis of polyarylethersulfone or polyaryletherketone that hassulfonic acid groups, there are mainly used a dihalide compound thatcontains fluorine and chlorine as halogen element. However, the dihalidecompound that has sulfonic acid groups and contains fluorine as halogenelement is expensive. For the synthesis of sulfonatedpolyarylethersulfone, in most cases, as the alkali metal salt of thedichloride having sulfonic acid groups, a sodium salt has beenspecifically used so far. However, the problem is that the blockcopolymer is hardly synthesized through ether-exchange reaction eventhough a hydrophilic segment prepolymer having a sodium salt as asulfonate group is synthesized using the sodium salt of the dichloridehaving sulfonic acid groups, and then block-copolymerized using thesynthesis.

The polymer electrolyte membrane described above is produced in anindustrial process as follows: a polymer electrolyte solution iscontinuously cast on a support and heated so as to evaporate the solventuntil a self-supporting membrane is formed; and then the self-supportingmembrane is peeled off from the support. As the support, a belt of ametal such as stainless steel is used in general. The polymerelectrolyte membrane having strong acid groups or superstrong acidgroups is not easy to peel off as a self-supporting membrane from themetal belt such as a stainless steel belt, and the membrane is sometimesbroken or scared during peeling off. Use of phosphate ester and/or saltbetween phosphate ester and amine as a separating agent is disclosed inthe method for adding to a solution of polyamide acid that is apolyimide precursor in Japanese Patent Laid-Open Publication No.S60-244507 (Patent Document 22). However, there are not any descriptionsabout the production of the polymer electrolyte membrane having strongacid groups such as sulfonic acid or phosphoric acid or superstrong acidgroups such as fluorinated alkylsulfonic acid.

Fuel cells installed in vehicles such as automobiles are required to besmall in weight and volume and cannot be attached with a largehumidification apparatus. Therefore, high proton conductivity isrequired under low humidity conditions. To improve the protonconductivity, increase of the ion-exchange capacity is effective, but tothe contrary there was a problem that the membrane properties onwater-absorption are lowered. In this way, a membrane having stillhigher proton conductivity has been requested among the membranes thathave the same ion-exchange capacity.

When the ion-exchange capacity is increased so as to maintain adequateproton conductivity, the membrane properties on water-absorption islowered. As a measure, a blend with a non-sulfonated polymer has beenproposed. For example, in Japanese Patent Laid-Open Publication No.H08-20716 (Patent Document 23), there is disclosed a composition that iscomposed of sulfonated polyetherketone, polysulfone and a hydrophilicpolymer; and also in Japanese Patent Laid-Open Publication No.2002-260687 (Patent Document 24), there is disclosed a composition thatis composed of polyimide and a block copolymer having sulfonic acidgroups. Further, in order to improve the adhesion to electrode layers, acomposition that is composed of an aromatic polyether and a sulfonatedaromatic polyether and has a flow start temperature of from 100° C. to220° C. is disclosed in Japanese Patent Laid-Open Publication No.2004-307629 (Patent Document 25). However, in the case of a blend of asulfonated homo-polymer or a random copolymer with a non-sulfonatedpolymer, and a blend of a block copolymer having sulfonic acid groupswith a non-sulfonated polymer that does not have the similar structureunit to the block copolymer, when the non-sulfonated polymer is blendedto an extent at which improved membrane properties on water-absorptionare attained, the proton conductivity is largely lowered at low humidityalthough there are not any problems on the proton conductivity at highhumidity. On the other hand, when the non-sulfonated polymer is blendedin an amount at which the proton conductivity at low humidity is notlowered largely, there was a problem that the membrane properties onwater-absorption was not sufficiently improved.

-   Patent Document 1: Japanese Patent Application Laid-Open No.    H11-502249-   Patent Document 2: Japanese Patent Application Laid-Open No.    2002-524631-   Patent Document 3: Japanese Patent Application Laid-Open No.    2000-510511-   Patent Document 4: Japanese Patent Laid-Open Publication No.    2003-64181-   Patent Document 5: Japanese Patent Laid-Open Publication No.    2003-68326-   Patent Document 6: U.S. Pat. No. 5,403,675-   Patent Document 7: Japanese Patent Laid-Open Publication No.    H10-21943-   Patent Document 8: Japanese Patent Laid-Open Publication No.    H10-45913-   Patent Document 9: Japanese Patent Laid-Open Publication No.    H11-116679-   Patent Document 10: Japanese Patent Laid-Open Publication No.    H11-67224-   Patent Document 11: Japanese Patent Laid-Open Publication No.    2002-60687-   Patent Document 12: Japanese Patent Laid-Open Publication No.    2004-190002-   Patent Document 13: Japanese Patent Laid-Open Publication No.    2004-190003-   Patent Document 14: Japanese Patent Laid-Open Publication No.    2004-346305-   Patent Document 15: Japanese Patent Laid-Open Publication No.    2002-358978-   Patent Document 16: Japanese Patent Laid-Open Publication No.    2003-234014-   Patent Document 17: Japanese Patent Application Laid-Open No.    2003-511510-   Patent Document 18: Japanese Patent Laid-Open Publication No.    2003-31232-   Patent Document 19: Japanese Patent Laid-Open Publication No.    2005-126684-   Patent Document 20: Japanese Patent Laid-Open Publication No.    2005-139432-   Patent Document 21: Japanese Patent Laid-Open Publication No.    2003-206354-   Patent Document 22: Japanese Patent Laid-Open Publication No.    S60-244507-   Patent Document 23: Japanese Patent Laid-Open Publication No.    H08-20716-   Patent Document 24: Japanese Patent Laid-Open Publication No.    2002-260687-   Patent Document 25: Japanese Patent Laid-Open Publication No.    2004-307629

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a sulfonated aromaticpolymer electrolyte that is inexpensive and durable and keeps highproton conductivity, an electrolyte membrane thereof, and a productionmethod and a use thereof.

Further, an object of the present invention is to provide a polymerelectrolyte composition that has improved properties on water-absorptionwithout lowering largely the proton conductivity at low humidity, amembrane that is composed of the polymer electrolyte composition, and afuel cell using them.

Furthermore, an object of the present invention is to provide aproduction method of a sulfonated polyarylether block copolymer that isinexpensive and durable and keeps high proton conductivity, and a blockcopolymer that is obtained by the production method.

Still further, an object of the present invention is to provide aproduction method of a polymer electrolyte membrane, wherein aself-supporting membrane is easily peeled off from a support such as ametal belt including a stainless steel belt when a membrane composed ofa polymer electrolyte having strong acid groups or superstrong acidgroups is produced.

The present inventors have made intensive studies to attain theaforementioned objectives and found that an electrolyte and anelectrolyte membrane that are composed of an aromatic block copolymerhaving a hydrophilic segment with sulfonic acid groups exhibit anextremely improved proton conductivity when the aromatic block copolymerhas an ion-exchange capacity in a specific amount or more. Thus, presentinvention has been accomplished based on these findings.

Further, the present inventors have found that a polymer electrolytecomposition that is composed of a sulfonated block copolymer (A) and anaromatic polymer (B) without sulfonic acid groups provides an improvedproperty on water-absorption while suppressing the lowing in the protonconductivity at low humidity that is accompanied by blending, whereinthe copolymer (A) and the polymer (B) each have aromatic rings in itsmain chain, and the copolymer (A) is composed of a hydrophilic segmenthaving sulfonic acid groups and a hydrophobic segment having no sulfonicacid groups. Thus, the present invention also has been accomplishedbased on this finding. The prevent inventors have also found that amembrane of the polymer electrolyte composition of the present inventionprovides an effect of increasing proton conductivity at low humidity ascompared with a membrane that is composed of only a sulfonated blockcopolymer having the similar structure to the copolymer used in thecomposition of the present invention, in the case where both membraneshave the same ion-exchange capacity. Thus, the present invention alsohas been accomplished based on this finding.

Still further, the present inventors have found that even a hydrophilicsegment prepolymer that is synthesized using a inexpensive sulfonatedaromatic dichloride compound can allow the block copolymerization with ahydrophobic segment prepolymer to proceed and provide a sulfonatedpolyarylether block copolymer by using a hydrophilic segment prepolymerhaving sulfonic acid groups in potassium salt form. Thus, the presentinvention also has been accomplished based on this finding.

Still further, the present inventors have found that a self-supportingmembrane is easily peeled off from a support after a polymer electrolytesolution that is admixed with a phosphate ester and/or salt betweenphosphate ester and amine is cast on a support and heated to dry, andfound that polymer electrolyte membrane can be produced continuously.Thus, the present invention also has been accomplished based on thisfinding.

Means for Solving the Problems

The present invention (hereinafter, referred to as “the first aspect”)ralats to a polymer electrolyte comprising a hydrophilic segment havingsulfonic acid groups and a hydrophobic segment having no sulfonic acidgroup, each segment having an aromatic ring in its main chain, whereinthe polymer electrolyte is an aromatic block copolymer having anion-exchange capacity IEC_(a) of solely the hydrophilic segment at 3.6mmol/g or more calculated from the following equation, and anion-exchange capacity IEC of the block copolymer in the range of from0.5 mmol/g to 3.0 mmol/g.

$\begin{matrix}{{IEC}_{a} = \frac{{IEC} \times W}{W_{a}}} & \left\lbrack {{Mathematical}\mspace{14mu} 1} \right\rbrack\end{matrix}$

wherein IEC is an ion-exchange capacity of the block copolymer, W is aweight of the block copolymer, and W_(a) is a weight of the hydrophilicsegment in the block copolymer.

Further, the present invention relates to a polymer electrolyte membranecomprising the polymer electrolyte of the first aspect, and having athickness of from 5 μm to 200 μm.

Still further, the present invention relates to a membrane/electrodeassembly comprising the polymer electrolyte membrane of the above.

Still further, the present invention relates to a solid polymerelectrolyte fuel cell comprising the polymer electrolyte of the firstaspect or the membrane/electrode assembly of the above.

Still further, the present invention relates to a direct liquid fuelinjection-type fuel cell using the polymer electrolyte of the firstaspect or the membrane/electrode assembly of the above, comprising usingalcohol or ether as a fuel.

Further, the present invention (hereinafter, referred to as “the secondaspect”) relates to a polymer electrolyte composition comprising: asulfonated block copolymer (A) composed of a hydrophilic segment havingsulfonic acid groups and a hydrophobic segment having no sulfonic group,each segment having an aromatic ring in its main chain; and an aromaticpolymer (B) having no sulfonic acid group and having the same structuralunit as a structural unit contained in a hydrophobic segment of asulfonated block copolymer; wherein a weight fraction P_(b) of thearomatic polymer (B) is in the range of from 0.03 to 0.4 calculated fromthe following equation.

$\begin{matrix}{P_{b} = \frac{W_{B}}{W_{A} + W_{B}}} & \left\lbrack {{Mathematical}\mspace{14mu} 2} \right\rbrack\end{matrix}$

wherein W_(A) is a weight of the sulfonated block copolymer (A); andW_(B) is a weight of the aromatic polymer (B).

Further, the present invention relates to a polymer electrolyte membranecomprising the polymer electrolyte composition of the second aspect, andhaving a thickness of from 5 μm to 200 μm.

Still further, the present invention relates to a membrane/electrodeassembly comprising using the polymer electrolyte composition of thesecond aspect.

Still further, the present invention relates to a solid polymerelectrolyte fuel cell comprising the polymer electrolyte composition ofthe second aspect or the membrane/electrode assembly.

Still further, the present invention relates to a direct liquid fuelinjection-type fuel cell using the polymer electrolyte composition ofthe second aspect or the membrane/electrode assembly, comprising usingalcohol or ether as a fuel.

Further, the present invention (hereinafter, referred to as “the thirdaspect”) relates to a method for producing a sulfonated polyaryletherblock copolymer, comprising producing a sulfonated polyarylether blockcopolymer containing a hydrophobic segment having a structural unitrepresented by the following formula (5) and a hydrophilic segmenthaving a structural unit having sulfonic acid groups or derivativethereof incorporated into a structure represented by the followingformula (6), wherein a hydrophilic segment prepolymer having sulfonicacid groups in a potassium salt form and a hydrophobic segmentprepolymer are block copolymerized.

wherein D¹ is SO₂ or CO; and Ar¹ is a divalent aromatic residue bondedan electron-withdrawing group in every aromatic ring belonging to Ar¹.

wherein, D² is SO₂ or CO; and Ar² is a divalent aromatic residue.

Further, the present invention relates to a sulfonated polyaryletherblock copolymer produced by the production method of the third aspect.

Still further, the present invention relates to a proton conductorcomprising using the above sulfonated polyarylether block copolymer.

Further, the present invention (hereinafter, referred to as “the forthaspect”) relates to a method for producing a polymer electrolytemembrane, comprising producing a polymer electrolyte membrane having astrong acid group or a superstrong acid group by casting, wherein apolymer electrolyte solution containing from 0.0005 part to 2 parts byweight of a phosphate ester represented by the following formula (11)and/or a salt between an amine represented by the following formula (12)and a phosphate ester represented by the following formula (11) withrespect to 100 parts by weight of a polymer electrolyte, is cast on asupport, heated until a solvent of the solution is evaporated to form aself-supporting membrane, and the self-supporting membrane is peeled offfrom the support.

wherein R¹ is a hydrogen atom, an alkyl group having 6 to 18 carbonatoms, or a group represented by the following formula (13); and R² isan alkyl group having 6 to 18 carbon atoms or a group represented by thefollowing formula (13),

[Formula 4]

R³—(OC₂H₄)_(m)—  (13)

wherein R³ is an alkyl group having 5 to 18 carbon atoms; and m is aninteger of from 2 to 30.

wherein R₄ to R₆ each are a hydrogen atom, a hydroxyethyl group, or analkyl group having 1 to 12 carbon atoms.

Further, the present invention relates to a polymer electrolyte membraneproduced by the method for producing a polymer electrolyte membrane ofthe forth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relative humidity dependence of proton conductivity ata measurement temperature of 50° C.

FIG. 2 shows the result of power generation test at a temperature of 70°C.

BEST MODE FOR CARRYING OUT THE INVENTION

Firstly, the first aspect of the present invention will be explained.

The hydrophilic segment of the sulfonated aromatic block copolymeraccording to the first aspect has an ion-exchange capacity IEC_(a) ofsolely the hydrophilic segment at 3.6 mmol/g or more, preferably 3.7mmol/g or more, and more preferably 3.8 mmol/g or more, as calculatedfrom the following equation. It is undesirable that the ion-exchangecapacity of the hydrophilic segment is lower than 3.6 mmol/g, becausethe proton conductivity is lowered.

$\begin{matrix}{{IEC}_{a} = \frac{{IEC} \times W}{W_{a}}} & \left\lbrack {{Mathematical}\mspace{14mu} 3} \right\rbrack\end{matrix}$

wherein IEC is an ion-exchange capacity of the block copolymer, W is aweight of the block copolymer, and W_(a) is a weight of the hydrophilicsegment in the block copolymer.

The ion-exchange capacity of the aforementioned block copolymer is inthe range of from 0.5 mmol/g to 3.0 mmol/g, preferably from 0.6 mmol/gto 2.9 mmol/g, and more preferably from 0.7 mmol/g to 2.8 mmol/g. It isundesirable that the ion-exchange capacity of the block copolymer islower than 0.5 mmol/g, because the proton conductivity becomes poor. Itis also undesirable that the ion-exchange capacity is higher than 3.0mmol/g, because the block copolymer becomes water-soluble or themembrane strength on water-absorption becomes largely lowered.

In the first aspect, the hydrophobic segment is a polymer segment thathas aromatic rings in its main chain considering heat-resistance, andmay include a segment of polyimide, polyphenylene, polyphenyleneoxide,polyphenylenesulfide, polyphenylenesulfoxide, polysulfone,polyethersulfone, polyetherketone, and others. Among these, a segment ofpolyethersulfone and/or polyetherketone represented by the followingformula (15) is preferable because the block copolymer can besynthesized easily. In particular, a segment that contains the structurerepresented by the following formula (1) is preferable. In view of cost,more preferable is a segment of the polyethersulfone that is given byselecting SO₂ for D¹ in the following formula (1):

wherein, D⁵ is CO or SO₂; Y⁵ is O or S; and Ar⁴ is a divalent aromaticresidue.

wherein D¹ is SO₂ or CO; Y¹ is O or S; and m is an integer of from 3 to1,500.

In the first aspect, the hydrophilic segment is incorporated withsulfonic acid groups in view of heat-resistance and has aromatic ringsin its main chain. Specifically there may be mentioned a segment ofpolyimde, polyphenylene, polyphenyleneoxide, polyphenylenesulfide,polyphenylenesulfoxide, polysulfone, polyethersulfone, polyetherketoneand others in which sulfonic acid groups are incorporated. The sulfonicacid groups may be incorporated directly into the aromatic rings of themain chain or may be bonded to the main chain through a C₁₋₁₂ alkyl orfluoroalkyl group, a C₆₋₂₄ aromatic residue, or a C₁₋₁₂ alkoxy group.From the viewpoint of the ease of synthesis of the block copolymer,preferable is a segment of polyethersulfone and/or polyether ketonehaving the structural unit that is given by incorporating sulfonic acidgroups into the structure represented by the following formula (2). Morepreferable is a segment of an aromatic polyethersulfone and/orpolyetherketone that is composed of a structural unit [A] represented bythe following formula (3) and a structural unit [B] represented by thefollowing formula (4) and has a weight ratio [A]/[B] of from 10/0 to1/9. In particular, in view of cost, preferable is a sulfonatedpolyethersulfone segment, which is given by selecting SO₂ for D³ and D⁴and O for Y³ and Y⁴ in the following formulas (3) and (4).

wherein D² is SO₂ or CO; Y² is O or S; and Ar¹ is a divalent aromaticresidue.

wherein D³ is SO₂ or CO; b is an integer of 0 or 1, and at least eitherone of b is 1; Z is a hydrogen atom or an alkali metal; Y³ is O or S;and Ar² is a divalent aromatic residue having sulfonic acid groups.

wherein D⁴ is SO₂ or CO; Y⁴ is O or S; and Ar³ is a divalent aromaticresidue having sulfonic acid groups.

There is not any particular limitation on the synthesis method of thesulfonated aromatic block copolymer used in the first aspect. Forexample, the copolymer can be synthesized by the following methods andthe like:

(1) A method in which a hydrophobic segment prepolymer and a hydrophilicsegment prepolymer that is not sulfonated or partly sulfonated each arepreliminary synthesized; then an un-sulfonated block copolymer or apartly sulfonated block copolymer is obtained; and the block copolymeris sulfonated after only the hydrophilic segment is further added;(2) A method in which a hydrophobic segment polymer and a sulfonatedhydrophilic segment polymer each are preliminary synthesized; and thenboth are reacted to obtain the block copolymer.

The hydrophobic segment prepolymer, and the prepolymers of sulfonatedhydrophilic segment, un-sulfonated hydrophilic segment and partlysulfonated segment (hereinafter, these are referred to as “hydrophilicsegment prepolymer”), which are used for the synthesis of the aromaticblock copolymer in the first aspect, can be synthesized bypublicly-known methods as described, for example, in “Shin KobunshiJikkengaku 3, Kobunshi No Gosei•Hanno (2), Shukugokei Kobunshi NoGosei”, edited by The Society of Polymer Science, Japan, published byKyoritsu Shuppan Co., Ltd.; Tokyo, 1996.

The polyethersulfone and/or polyetherketone that are the hydrophobicsegment prepolymer and hydrophilic segment polypolymer used for thesynthesis of the preferable aromatic polyether block copolymer in thefirst aspect can be synthesized by reacting a di-alkali metal salt of adihydric phenol compound and an aromatic dihalide compound as disclosed,for example, in “Shin Kobunshi Jikkengaku 3, Kobunshi No Gosei Hanno(2), Shukugokei Kobunshi No Gosei”, edited by The Society of PolymerScience, Japan, published by Kyoritsu Shuppan Co., Ltd.; Tokyo, 1996; R.N. Johnson et al., J. Polym. Sci., A-1, Vol. 5, p. 2375 (1967); orJapanese Examined Patent Publication No. S46-21458.

The aromatic dihalide compound that is used for the synthesis of thepreferable polyether block copolymer in the first aspect may include,for example, bis(4-chlorophenyl)sulfone, bis(4-fluorophenyl)sulfone,bis(4-bromophenyl) sulfone, bis(4-iodephenyl)sulfone,bis(2-chlorophenyl)sulfone, bis(2-fluorophenyl) sulfone,bis(2-methyl-4-chlorophenyl)sulfone, bis(4-chlorophenyl)ketone,bis(4-fluorophenyl)ketone, bis(4-bromophenyl)ketone,bis(4-iodephenyl)ketone, bis(2-chlorophenyl)ketone,bis(2-fluorophenyl)ketone, bis(2-methyl-4-chlorophenyl)ketone and thelike. These may be used alone or in a combination of two or more kinds.Among these, there may be mentioned, preferablybis(4-chlorophenyl)sulfone, bis(4-fluorophenyl)sulfone,bis(4-chlorophenyl)ketone, and bis(4-fluorophenyl)ketone, and morepreferably bis(4-chlorophenyl)sulfone and bis(4-fluorophenyl)sulfone. Inaddition, if necessary, 2,6-dichlorobenzonitrile and2,6-difluorobenzonitrile may be used.

Further, besides the aforementioned aromatic dihalide compounds, as thesulfonated aromatic dihalide compounds that are used as a sourcematerial for forming the hydrophilic segment, there may be used acompound that is given by incorporating one or two sulfonic acid groupsand/or their alkali metal salts into the aforementioned aromaticdihalide compounds. The compound may include, for example,3,3′-disulfo-4,4′-dichlorodiphenylsulfone,3,3′-disulfo-4,4′-difluorodiphenylsulfone,3-sulfo-4,4′-dichlorodiphenylsulfone,3-sulfo-4,4′-difluorodiphenylsulfone,3,3′-disulfo-4,4′-dichlorodiphenylketone,3,3′-disulfo-4,4′-difluorodiphenylketone,3-sulfo-4,4′-dichlorodiphenylketone, 3-sulfo-4,4′-difluorodiphenylketoneand the like, and/or their alkali metal salts such as lithium, sodiumand potassium salts. These compounds may be used alone or in acombination of two or more kinds. Among these, there may be mentioned,preferably 3,3′-disulfo-4,4′-dichlorodiphenylsulfone,3,3′-disulfo-4,4′-difluorodiphenylsulfone,3-sulfo-4,4′-dichlorodiphenylsulfone,3-sulfo-4,4′-difluorodiphenylsulfone and the like, and/or their alkalimetal salts such as lithium, sodium and potassium salts.

The dihydric phenol that is used for the synthesis of the preferablepolyether block copolymer in the first aspect may include, for example,hydroquinone, resorcinol, 1,5-dihydroxynaphthalene,1,6-dihydroxynaphthalene, 1,7-dihydroxynaphthalene,2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 4,4′-biphenol,2,2′-biphenol, 2,4′-biphenol, bis(4-hydroxyphenyl)ether,bis(2-hydroxyphenyl)ether, 2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-methyl-4-hydroxyphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl) propane,bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)sulfone,bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)ketone,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)hexafluoropropane,9,9-bis(4-hydroxyphenyl)fluorene, 1,3-bis(4-hydroxyphenoxy)benzene,1,4-bis(4-hydroxyphenoxy)benzene and the like. These may be used aloneor in a combination of two or more kinds.

In the first aspect, when further sulfonation is carried out after theblock copolymer is synthesized, in order to allow the sulfonation toproceed selectively, it is desirable that an electron-withdrawing groupsuch as ketone or sulfone is bonded to each aromatic ring of thedihydric phenol used for the synthesis of the hydrophobic segmentprepolymer so as to allow the dihydric phenol to have a structure noteasily sulfonated. Such dihydric phenol may include preferablybis(4-hydroxyphenyl)sulfone and bis(4-hydroxyphenyl)ketone among theaforementioned dihydric phenols.

On the other hand, when further sulfonation is carried out after theblock copolymer is synthesized, in order to allow the sulfonation toproceed selectively, it is desirable that an electron-withdrawing groupsuch as ketone or sulfone is not bonded to each aromatic ring of thedihydric phenol used for the synthesis of the hydrophilic segmentprepolymer so as to allow the dihydric phenol to have a structure easilysulfonated. Such dihydric phenol may include preferably, for example,hydroquinone, resorcinol, 1,4-dihydroxynaphthalene,1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene,1,7-dihydroxynaphthalene, 2,6-dihydroxynaphthalene,2,7-dihydroxynaphthalene, 4,4′-biphenol, 2,2′-biphenol,bis(4-hydroxyphenyl)ether, bis(2-hydroxyphenyl)ether,2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-methyl-4-hydroxyphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)sulfide,2,2-bis(3,5-dimethyl-4-hydroxyphenyl) hexafluoropropane,9,9-bis(4-hydroxyphenyl)fluorene among the aforementioned dihydricphenols and the like. From the viewpoint of reactivity, there may bementioned preferably hydroquinone, resorcinol, 1,5-dihydroxynaphthalene,2,7-dihydroxynaphthalene, 4,4′-biphenol, bis(4-hydroxyphenyl)ether,2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-methyl-4-hydroxyphenyl)propane, bis(4-hydroxyphenyl)methane,and 9,9-bis(4-hydroxyphenyl)fluorene.

When each segment prepolymer is synthesized, molecular weight may bemodified and/or the terminal groups that are used for the synthesis ofthe block copolymer may be formed by using excessively either one of theaforementioned dihydric phenols or aromatic dihalide compounds.Alternatively, when the dihydric phenols and aromatic dihalides are usedin equi-molar with each other, in order to modify the molecular weightand to form the terminal groups required for the synthesis of the blockcopolymer, may be added either one of a monohydric phenol such asphenol, cresol, 4-phenylphenol and 3-phenylphenol, and an aromatichalide such as 4-chlorophenylphenylsulfone, 1-chloro-4-nitrobenzene,1-chloro-2-nitrobenzene, 1-chloro-3-nitrobenzene, 4-flulorobenzophenone,1-fluoro-4-nitrobenzene, 1-fluoro-2-nitrobenzene and1-fluoro-3-nitrobenzene.

The polymerization degree of each segment prepolymer is in the range offrom 3 to 1,500, and preferably from 5 to 1,000. At a polymerizationdegree of lower than 3, the block copolymer synthesized using aprepolymer becomes difficult to exhibit adequate properties. On theother hand, when the polymerization degree exceeds 1,500, the blockcopolymer becomes difficult to synthesize.

In the first aspect, when further sulfonation is carried out after theblock copolymer is synthesized, in order to sulfonate selectively onlythe hydrophilic segment, it is desirable that an electron-withdrawinggroup is bonded to the aromatic rings that form the hydrophobic segment.In this case, the most preferable hydrophobic segment prepolymer has astructure represented by the following formula (16):

wherein, m is an integer of from 3 to 1,500.

As the hydrophobic segment prepolymer and un-sulfonated hydrophilicsegment prepolymer, commercially available polymers having the adequatestructures may be used. The commercially available polymers may be usedafter the molecular weight and the terminal groups are modified byether-exchange reaction with the aforementioned dihydric alkali metalsalt or a monohydric phenol describing below under similar conditions tothe polyethersulfone synthesis as described in R. N. Johnson et al., J.Polym. Sci., A-1, Vol. 5, p. 2375 (1967) or Japanese Patent ApplicationPublication No. S46-21458.

In the case of synthesizing the sulfonated aromatic polyether blockcopolymer used in the first aspect with the procedure in whichun-sulfonated or partly sulfonated block copolymer is synthesized andthen further sulfonated, the un-sulfonated or partly sulfonated blockcopolymer can be synthesized by reacting the aforementioned hydrophobicsegment prepolymer that has halogen terminal groups or alkali metal saltterminal groups with the aforementioned hydrophilic segment prepolymerthat has terminal groups corresponding to the terminal groups of thehydrophobic segment prepolymer, using the method as described in Z. Wuet al., Angew. Makromol. Chem., Vol. 173, p. 163 (1989); Z. Wu et al.,Polym. Int., Vol. 50, p. 249 (2001); and others. In addition, bothun-sulfonated and partly sulfonated block copolymers can be synthesizedfrom the segment prepolymers that have phenol alkali-metal salt terminalgroups using a linking agent in the same manner as mentioned above. Asthe linking agent, there may be mentioned the aromatic dihalidesdescribed above, preferably an aromatic dihalide that contains fluorineas halogen and has a high reactivity. Further, in the case of apolyethersulfone/polythioethersulfone block copolymer, the blockcopolymer can be also synthesized using the method disclosed in JapanesePatent Laid-Open Publication No. S61-168629. Still further, in the caseof using the hydrophobic segment prepolymer represented by theabove-mentioned formula (2), as disclosed in Japanese Patent Laid-OpenPublication No. 2003-206354, the block copolymer can be synthesized byreacting the hydrophilic segment prepolymer that has phenol alkali-metalsalt terminal groups with a commercially available hydrophobic segmentprepolymer in a solution at a temperature of from 120° C. to 200° C.

The method of further sulfonating the un-sulfonated or partly sulfonatedblock copolymer has been known. The resulting un-sulfonated or partlysulfonated block copolymer is reacted, for example as described inJapanese Patent Laid-Open Publication No. S61-36781, Japanese PatentApplication Publication No. H01-54323, Japanese Patent ApplicationPublication No. H02-17571 and others, in a concentrated sulfuric acid of95 wt % to 98 wt %, for 0.2 hour to 96 hours, at 10° C. to 80° C., sothat only the hydrophilic segment is sulfonated, whereby the sulfonatedaromatic block copolymer of the present invention can be obtained.

In the case of synthesizing the sulfonated aromatic ether blockcopolymer used in the first aspect from a sulfonated hydrophilic segmentprepolymer and a hydrophobic segment prepolymer, the aforementionedun-sulfonated hydrophilic segment prepolymer may be reacted tosynthesize the sulfonated aromatic polyester block copolymer with thehydrophilic segment using a sulfonated prepolymer in the same manner asused in the synthesis of the aforementioned un-sulfonated blockcopolymer.

The sulfonated aromatic block copolymer of the first aspect has ahydrophilic segment weight fraction F_(a) as calculated from thefollowing equation of preferably from 0.1 to 0.8, and more preferablyfrom 0.2 to 0.7. The fraction smaller than 0.1 is undesirable, becausethe proton conductivity becomes lowered. The fraction larger than 0.8 isalso undesirable, because the block copolymer becomes water-soluble.

$\begin{matrix}{F_{a} = \frac{W_{a}}{W_{a} + W_{b}}} & \left\lbrack {{Mathematical}\mspace{14mu} 4} \right\rbrack\end{matrix}$

wherein F_(a) is hydrophilic segment weight fraction; W_(a) ishydrophilic segment weight; and W_(b) is hydrophobic segment weight.

In the first aspect, the polymer electrolyte of the aromatic blockcopolymer or a polymer electrolyte membrane obtained from the aromaticblock copolymer has a proton conductivity at 50° C. and a relativehumidity of 90% of preferably 1×10⁻² S/cm or more, and particularlypreferably 1.5×10⁻² S/cm or more. It is undesirable that the protonconductivity is lower than 1×10⁻² S/cm, because power generationperformance is lowered. Further, in the first aspect, the foregoingpolymer electrolyte or membrane thereof has a proton conductivity at 50°C. and a relative humidity of 40% of preferably 4×10⁻³ S/cm or more,more preferably 5×10⁻³ S/cm or more, and particularly preferably 6×10⁻³S/cm or more. Still further, the foregoing polymer electrolyte ormembrane thereof has a proton conductivity at 70° C. and a relativehumidity of 30% of preferably 2×10⁻³ S/cm or more, more preferably2.5×10⁻³ S/cm or more, and particularly preferably 3×10⁻³ S/cm or more.

There is not any particular limitation on the method of forming thesulfonated aromatic block copolymer obtained as described above into thepolymer electrolyte membrane of the first aspect. For example, thesulfonated aromatic block copolymer is dissolved in a solvent, and thenthe resulting solution is cast on a support and heated to remove thesolvent by evaporation to obtain a membrane. The solvent is not requiredto be fully removed from the membrane on the support, but the membraneis peeled off from the support after a self-supporting membrane isformed, and then the solvent may be removed by heating.

For example, the preferred sulfonated aromatic polyether block copolymerin the first aspect is dissolved in a polar solvent such asdimethylsulfoxide, sulfolane, N-methyl-2-pyrolidone,1,3-dimethyl-2-imidazolidinone, N,N-dimethylformamide,N,N-dimethylacetoamide and diphenylsulfone; after the resulting solutionis cast on a support, the solution is dried at 80° C. to 250° C. for 0.5minute to 48 hours to remove the polar solvent by evaporation, whereby amembrane can be obtained. Alternatively, when a self-supporting membraneis developed, the membrane may be peeled off and further dried at 80° C.to 250° C. for 0.5 minute to 48 hours to remove the polar solvent byevaporation. The drying temperature lower than 80° C. is undesirable,because the membrane is not fully dried. Above 250° C. is alsoundesirable, because the membrane is possibly decomposed.

The polymer electrolyte membrane of the first aspect has a thickness offrom 5 μm to 200 μm, and preferably from 10 μm to 150 μm. The thicknessless than 5 μm is undesirable, because the membrane becomes difficult tohandle. The thickness more than 200 μm is also undesirable, because thepower generation efficiency of fuel cells lowers.

A phase separation structure is found in the polymer electrolytemembrane of the first aspect when the cross-section thereof is observedwith a transmission electron microscope at a magnification of 90,000times, wherein the phase separation structure has an average distancebetween domains or lamellas of from 5 nm to 900 nm, and preferably from10 nm to 800 nm. The polymer electrolyte of the first aspect exhibits anexcellent proton conductivity at low humidity. This is possibly becauseproton conduction paths arising from the hydrophilic segment are formedby the phase separation structure, and sulfonic acid groups are presentat a high density in the area where the hydrophilic segment forms.

The polymer electrolyte membrane of the first aspect, if necessary, maybe allowed to have sulfonic acid groups that are partly converted tometal salts as long as the properties of the first aspect are notimpaired. Further, the polymer electrolyte membrane may be reinforcedwith fibers, porous membranes, and others. Still further, if necessary,can be blended an inorganic acid such as phosphoric acid,hypophosphorous acid and sulfuric acid, or their salts, C₁₋₁₄perfluoroalkylsulfonic acids or their salts, C₁₋₁₄perfluoroalkylcarboxylic acids or their salts, an inorganic materialsuch as platinum, silica gel, silica and zeolite, and the otherpolymers.

There is not any particular limitation on the production method of afuel cell using the polymer electrolyte membrane of the first aspect.The fuel cell may be produced by known methods in the art, namely, thefuel cell is produced by bonding a catalyst and a conductive materialthat serves as a current collector on both sides of the polymerelectrolyte membrane of the aromatic block copolymer. The aromatic blockcopolymer of the first aspect can be also used as an ion conductivecomponent for a catalyst layer. That is, the aromatic block copolymer ofthe first aspect can be used not only as the electrolyte membrane of apolymer electrolyte membrane/electrode assembly used for a fuel cell,but also as an ion conductive component for the catalyst layer. Thearomatic block copolymer can be used for both membrane and catalystlayer and provide a membrane/electrode assembly.

There is not any particular limitation on the catalyst, but can be useda publicly-known catalyst as long as the catalyst can activate the redoxreaction of hydrogen or oxygen. For example, fine particles of platinumor platinum alloys can be used. The platinum fine particles aresupported on granular or fibrous carbon such as active carbon andgraphite.

As the conductive material serving as a current collector, can be alsoused a publicly-known material. For example, a porous woven fabric ofcarbon, a nonwoven fabric of carbon and a carbon paper are preferable,because source gases can be transported efficiently to the catalyst.

As the method of bonding the platinum fine particles or the carbonmaterial loaded with platinum fine particles to the porous nonwovenfabric of carbon or carbon paper, and also the method of bonding theresulting catalyst layer to a polymer electrolyte film, can be usedpublicly-known methods that are described, for example, in J.Electrochem. Soc.: Electrochemical Science and Technology, Vol. 13105,No. 9, p. 2209 (1988) and the like.

Thus produced fuel cell of the first aspect can be used in a variety ofconfigurations using as a fuel, hydrogen gas, reformed hydrogen gas,alcohol, ether or the like.

Next, the second aspect of the present invention will be explained.

The sulfonated block copolymer (A) used in the second aspect is composedof a hydrophilic segment having sulfonic acid groups and a hydrophobicsegment having no sulfonic acid groups, both segments each havingaromatic rings in its main chain. Both segments contain as a component,for example, an aromatic polymer structure including a polyimide; anaromatic polyether such as polyethersulfone, polysulfone,polyetherketone, polyetheretherketone, polyetherketoneketone andpolyphenyleneoxide; a polyarylene; a polysulfide such aspolyphenylenesulfide; and a polyazole such as polybenzimidazole,polybenzoxazole and polybenzothiazole. The component that forms thehydrophilic segment and hydrophobic segment may be the same ordifferent. Further, each segment can be composed of two or more kinds ofcomponents. The sulfonic acid groups of the hydrophilic segment may bedirectly incorporated into the aromatic rings of the main chain, orbonded to the main chain through a C₁₋₁₂ alkyl or fluoroalkyl group, aC₆₋₂₄ aromatic residue, a C₁₋₁₂ alkoxy group, and others. The sulfonatedblock copolymer has an ion-exchange capacity of preferably from 0.6mmol/g to 3.0 mmol/g, more preferably from 0.8 mmol/g to 2.8 mmol/g,still more preferably from 1.0 mmol/g to 2.7 mmol/g, and particularlypreferably from 1.2 mmol/g to 2.6 mmol/g. In order to attain a highproton conductivity at low humidity, the sulfonated block copolymerparticularly preferably has such characteristics that the hydrophilicsegment of the sulfonated block copolymer has an ion-exchange capacityIEC_(W) of 3.6 mmol/g or more and the sulfonated block copolymer has anion-exchange capacity IEC of from 0.6 mmol/g to 3.0 mmol/g. It isundesirable that the ion-exchange capacity of the sulfonated blockcopolymer is lower than 0.6 mmol/g, because the proton conductivity islowered. It is also undesirable that the ion-exchange capacity is largerthan 3.0 mmol/g, because the sulfonated block copolymer becomeswater-soluble.

$\begin{matrix}{{IEC}_{w} = \frac{{IEC} \times W}{W_{w}}} & \left\lbrack {{Mathematical}\mspace{14mu} 5} \right\rbrack\end{matrix}$

wherein IEC_(W) is an ion-exchange capacity of the sulfonated blockcopolymer; W is a weight of the sulfonated block copolymer; and W_(W) isa weight of the hydrophilic segment in the sulfonated block copolymer.

Among the aforementioned structures, the sulfonated block copolymerwhose both segments are selected from an aromatic polyether, polyaryleneand polysulfide is preferable in view of water-resistance, and in viewof ease of synthesis more preferably both segments are an aromaticpolyether.

In the second aspect, as the preferable hydrophilic segment of thearomatic polyether sulfonated block copolymer, there may be mentionedthe same preferable hydrophilic segment as that exemplified in thearomatic block copolymer of the first aspect.

In the second aspect, as the preferable hydrophobic segment of thearomatic polyether sulfonated block copolymer, there may be mentionedthe same preferable hydrophobic segment as that exemplified in thearomatic block copolymer of the first aspect.

There is not any particular limitation on the synthesis method of thesulfonated block copolymer used in the second aspect, but the blockcopolymer can be synthesized by the same method as the above-exemplifiedsynthesis method of the aromatic block copolymer in the first aspect andthe like.

The sulfonated block copolymer of the second aspect has a hydrophilicsegment weight fraction of preferably from 0.1 to 0.8, and morepreferably from 0.2 to 0.7 similar to that in the aromatic blockcopolymer of the first aspect. Still more preferably the fraction is inthe range of from 0.3 to 0.6. The fraction of smaller than 0.1 isundesirable since the proton conductivity becomes lowered. On the otherhand, the fraction of larger than 0.8 is undesirable since the blockcopolymer becomes water-soluble.

The reason why the composition of the second aspect exhibit an adequateproton conductivity at low humidity as well as adequate properties onwater-absorption is that the non-sulfonated aromatic polymer has thesame structural unit as the hydrophobic segment of the sulfonated blockcopolymer, and that an adequate miscibility and a desirable phaseseparation structure can be attained.

As the non-sulfonated aromatic polymer (B) used in the second aspect,there may be mentioned the same polymers that are exemplified asspecific examples for both segments of the aforementioned sulfonatedblock copolymer. The polymer (B) does not contain sulfonic acid groupsand contains the same structural unit as that contained in thehydrophobic segment of the sulfonated block copolymer. The polymer (B)may have a structure of homo-polymer, random copolymer, or blockcopolymer. In the case of a random or block copolymer, it is desirablethat the same structural unit as that contained in the hydrophobicsegment of the non-sulfonated aromatic polymer is contained in an amountof preferably 30 wt % or more, more preferably 35 wt % or more, andstill more preferably 40 wt % or more. A block copolymer that has asegment having the same structure as that in the hydrophobic segment, oran aromatic polymer that has the same structure as that in thehydrophobic segment is preferable. For example, when a sulfonated blockcopolymer having the hydrophobic segment represented by theabove-mentioned formula (1) is used, a block copolymer or a homo-polymerthat has the segment represented by the same formula as formula (1) ispreferably used. In the case of a block copolymer, as the structure ofthe other segments there may be used the structure that is exemplifiedas a specific example for both segments of the aforementioned sulfonatedblock copolymer. The kind of the segments may be one or two or more.

In the polymer electrolyte composition of the second aspect, the weightfraction P_(b) of aromatic polymer (B) is in the range of from 0.03 to0.4, preferably from 0.03 to 0.3, and more preferably from 0.05 to 0.25as calculated from the following equation. It is undesirable that theweight fraction P_(b) is lower than 0.03, because the properties onwater-absorption become lowered. It is also undesirable that P_(b) islarger than 0.4, because the proton conductivity at low humidity becomeslowered.

$\begin{matrix}{P_{b} = \frac{W_{B}}{W_{A} + W_{B}}} & \left\lbrack {{Mathematical}\mspace{14mu} 6} \right\rbrack\end{matrix}$

wherein W_(A) is a weight of the sulfonated block copolymer (A); andW_(B) is a weight of the aromatic polymer (B).

The non-sulfonated aromatic polymer used in the present inventiondesirably has such a molecular weight that the polymer can exhibit afilm-forming ability by itself, from the viewpoint of improving theproperties on water-absorption.

The polymer electrolyte composition of the second aspect has anion-exchange capacity of preferably from 0.5 mmol/g to 2.9 mmol/g, andmore preferably from 0.7 mmol/g to 2.5 mmol/g. It is undesirable thatthe ion-exchange capacity is smaller than 0.5 mmol/g, because the protonconductivity becomes lowered. It is also undesirable that theion-exchange capacity is larger than 2.9 mmol/g, because the propertieson water-absorption become lowered.

In the second aspect, the composition is prepared by thesolution-blending process in which a solvent capable of dissolving boththe sulfonated block copolymer and non-sulfonated aromatic polymer isused. There is not any particular limitation on the solvent as long asthe solvent can dissolve both components. As the solvent, there may bementioned, for example, a polar solvent such as dimethylsulfoxide,sulfolane, N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone,N,N-dimethylformamide, N,N-dimethylacetoamide, diphenylsulfone, phenol,m-cresol and p-chlorophenol, and a halogenated solvent such asdichloromethane, 1,2-dichloroethane and chloroform. Blending can becarried out by dissolving uniformly in a solvent both components insolid state in predetermined amounts, or by mixing the non-sulfonatedaromatic polymer with a solution of the sulfonated block copolymerobtained after the synthesis thereof.

Further, in the case where an electron-withdrawing group such as ketone,sulfone, nitro, cycano and trifluoromethyl, or a halogen such aschlorine, fluorine and bromine is bonded to substantially almost all ofthe aromatic rings of the non-sulfonated aromatic polymer, thecomposition may be prepared by dissolving both components together inconcentrated sulfuric acid in the sulfonation process of theaforementioned un-sulfonated or partly sulfonated block copolymer. Stillfurther, the composition may be prepared by dissolving both componentsin the prepared solution of the un-sulfonated or partly sulfonated blockcopolymer.

The sulfonated block copolymer used for these blending processes mayhave sulfonic acid groups that are not only in an acid form but also ina salt form with an organic base such as an aliphatic amine includingammonia, triethylamine, tripropylamine and tributylamine and aheterocyclic ring-containing base including imidazole, pyridine,quinoline or the like, or an alkali metal such as potassium, sodium andlithium.

The polymer electrolyte membrane of the second aspect is obtained bycasting a solution of the polymer electrolyte composition on a support,heating and evaporating the solvent until a self-supporting membrane isformed; then peeling off the resulting self-supporting membrane from thesupport, if necessary further heating the membrane. Thus obtainedpolymer electrolyte membrane may be used as it is, but it is desirablethat the polymer electrolyte membrane is further treated with an acidaqueous solution depending on applications such as fuel cells. Inaddition, if necessary, before the treatment with acid aqueous solutionthe membrane may be treated with an alkali aqueous solution. Note that,in the case of the sulfonated block copolymer having polyimidestructure, the treatment with the alkali aqueous solution is notdesirable because the membrane is possibly hydrolyzed, so that themembrane is desirably treated only with the acid aqueous solution. Ineach treatment, the membrane may be immersed in each aqueous solutionfor from 0.05 minute to 600 minutes in batch-wise or continuously. Shortimmersion is undesirable, because the treatment becomes insufficient.Long immersion is also undesirable, because no further change in theeffect can be expected. After each treatment, if necessary,water-washing process and/or drying process may be incorporated as, forexample, (1) alkali treatment→(2) water washing→(3) acid treatment→(4)water washing→(5) drying. Still further, if necessary, a process oftreating with alcohols such as methanol, ethanol and propanol, orketones such as acetone can be incorporated.

For the alkali treatment may be used an aqueous solution prepared bydissolving NaOH, KOH, sodium carbonate, potassium carbonate, sodiumhydrogen carbonate and potassium hydrogen carbonate in water. For theacid treatment, an aqueous solution prepared by dissolving hydrochloricacid, sulfuric acid and phosphoric acid in water may be used. Eachsolution is used in the range of from 0.01 N to 0.5 N of concentration.When the concentration of the aqueous solution is lower than 0.01 N, thetreatment becomes possibly insufficient. At a concentration of higherthan 5 N, the handling of the aqueous solution may become undesirablydifficult. Either aqueous solution treatment is carried out at atemperature of from 5° C. to 90° C. When the temperature is lower than5° C., the treatment becomes possibly insufficient. At a temperature ofhigher than 90° C., the handling of the aqueous solution may becomeundesirably difficult. The water-washing process may employ either ofimmersion or shower washing, or both in combination. The water-washingprocess is carried out at the same temperature and time as those of theaforementioned alkali or acid treatment. There is not any particularlimitation on the drying process, but, for example, water is evaporatedat a temperature of from room temperature to 200° C.

The polymer electrolyte membrane of the second aspect has a thickness offrom 5 to 200 μm, and preferably from 10 to 150 μm. At a thickness lessthan 5 μm, the membrane is not easy to handle. The membrane thicker than200 μm provides a fuel cell with undesirably low power generationefficiency.

In the second aspect, at 70° C. and 30% RH, the ratio C₂/C₁ of theproton conductivity C₁ of the membrane that is composed of only thesulfonated block copolymer to the proton conductivity C₂ of the membranethat is composed of the composition is preferably 0.5 or more, and morepreferably 0.6 or more. Further, the proton conductivity at 70° C. and30% RH is preferably 1 mS/cm or more, and particularly preferably 2mS/cm or more.

In the second aspect, at 23° C. in the water, the ratio E₂/E₁ (retentionratio) of the elasticity E₁ of the membrane that is composed of only thesulfonated block copolymer to the elasticity E₂ of the membrane that iscomposed of the composition is preferably 1.1 or more, and morepreferably 1.2 or more. Still further, at 23° C. in the water, the ratioS₂/S₁ (retention ratio) of the breaking strength S₁ of the membrane thatis composed of only the sulfonated block copolymer to the breakingstrength S₂ of the membrane that is composed of the composition ispreferably 1.0 or more, and more preferably 1.1 or more.

The polymer electrolyte composition or the polymer electrolyte membraneof the second aspect, if necessary, may be allowed to have sulfonic acidgroups that are partly converted to alkali metal salts or theaforementioned organic base salts as long as the properties of the firstaspect are not impaired. Further, the polymer electrolyte membrane maybe reinforced with fibers, porous membranes, and others. Still further,if necessary, can be blended an inorganic acid such as phosphoric acid,hypophosphorous acid and sulfuric acid, or their salts, C₁₋₁₄perfluoroalkylsulfonic acids or their salts, C₁₋₁₄perfluoroalkylcarboxylic acids or their salts, an inorganic materialsuch as platinum, silica gel, silica and zeolite, and the otherpolymers.

There is not any particular limitation on the production method of afuel cell using the polymer electrolyte membrane of the second aspect,but there may be mentioned the same method as that of the productionmethod of the fuel cell using the polymer electrolyte membrane of thefirst aspect.

Thus produced fuel cell of the second aspect can be used in a variety ofconfigurations using as a fuel, hydrogen gas, reformed hydrogen gas,alcohol, ether or the like.

Next, the third aspect of the present invention will be explained. Theproduction method of the sulfonated polyarylether block copolymer of thethird aspect is suitably applied to the production of the aromatic blockcopolymer of the first aspect and the sulfonated block copolymer (A) ofthe second aspect.

The hydrophilic segment prepolymer used in the third aspect issynthesized by nucleophilic substitution reaction between an alkalimetal salt such as sodium, potassium and lithium salt of an aromaticdihalide containing sulfonic acid group and a dihydric phenol compound.The aromatic dihalide containing sulfonic acid group used for thereaction may include, for example,3,3′-disulfo-4,4′-dichlorodiphenylsulfone,3,3′-disulfo-4,4′-difluorodiphenylsulfone,3-sulfo-4,4′-dichlorodiphenylsulfone,3-sulfo-4,4′-difluorodiphenylsulfone,3,3′-disulfo-4,4′-dichlorodiphenylketone,3,3′-disulfo-4,4′-difluorodiphenylketone,3-sulfo-4,4′-dichlorodiphenylketone, 3-sulfo-4,4′-difluorodiphenylketoneand the like. In view of reactivity,3,3′-disulfo-4,4′-dichlorodiphenylsulfone,3,3′-disulfo-4,4′-difluorodiphenylsulfone,3-sulfo-4,4′-dichlorodiphenylsulfone, and3-sulfo-4,4′-difluorodiphenylsulfone are preferable. Further, in view ofcost, 3,3′-disulfo-4,4′-dichlorodiphenylsulfone and3-sulfo-4,4′-dichlorodiphenylsulfone are the most preferable. Thesearomatic dihalide compounds containing sulfonic acid group may be usedalone or in a combination of two or more kinds. As an alkali metal thatforms a salt with the sulfonic acid groups, there may be mentioned theabove described ones. Among them, potassium is the most preferable,because the resulting potassium salt can be used as it is withoutfurther substitution.

In the third aspect, the dihydric phenol that is used for the synthesisof the hydrophilic segment prepolymer may include, for example,hydroquinone, resorcinol, 1,5-dihydroxynaphthalene,1,6-dihydroxynaphthalene, 1,7-dihydroxynaphthalene,2,7-dihydroxynaphthalene, 4,4′-biphenol, 2,2′-biphenol, 2,4′-biphenol,bis(4-hydroxyphenyl)ether, bis(2-hydroxyphenyl)ether,2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-methyl-4-hydroxyphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)sulfone,bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl) ketone,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)hexafluoropropane,9,9-bis(4-hydroxyphenyl)fluorene, 1,3-bis(4-hydroxyphenoxy)benzene,1,4-bis(4-hydroxyphenoxy)benzene and the like. A dihydric phenol havingno electron-withdrawing groups such as sulfone or ketone bonded to thearomatic rings is preferable, because the dihydric phenol is capable ofbeing sulfonated after the synthetic reaction of the block copolymerdescribing below and after the synthesis of the block copolymer. Such acompound may include, for example, hydroquinone, resorcinol,1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene,1,7-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 4,4′-biphenol,2,2′-biphenol, 2,4′-biphenol, bis(4-hydroxyphenyl)ether,bis(2-hydroxyphenyl)ether, 2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3-methyl-4-hydroxyphenyl)propane,2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane,bis(4-hydroxyphenyl)methane, 9,9-bis(4-hydroxyphenyl)fluorene,1,3-bis(4-hydroxyphenoxy)benzene, 1,4-bis(4-hydroxyphenoxy)benzene andthe like. These dihydric phenols may be used alone or in a combinationof two or more kinds.

In the third aspect, besides the aromatic dihalide compound containingsulfonic acid group, an aromatic dihalide compound having no sulfonicacid groups can be used. There may be mentioned, for example,bis(4-chlorophenyl)sulfone, bis(4-fluorophenyl)sulfone,bis(4-bromophenyl)sulfone, bis(4-iodephenyl)sulfone,bis(2-chlorophenyl)sulfone, bis(2-fluorophenyl)sulfone,bis(2-methyl-4-chlorophenyl)sulfone, bis(4-chlorophenyl)ketone,bis(4-fluorophenyl)ketone, bis(4-bromophenyl)ketone,bis(4-iodephenyl)ketone, bis(2-chlorophenyl)ketone,bis(2-fluorophenyl)ketone, bis(2-methyl-4-chlorophenyl)ketone, andothers. These may be used alone or in a combination of two or morekinds. Among these, preferable are bis(4-chlorophenyl)sulfone,bis(4-fluorophenyl)sulfone, bis(4-chlorophenyl) ketone, andbis(4-fluorophenyl)ketone. Particularly preferable arebis(4-chlorophenyl)sulfone and bis(4-fluorophenyl)sulfone. In addition,if necessary, 2,6-dichlorobenzonitrile and 2,6-difluorobenzonitrile maybe used. These aromatic dihalide compounds having no sulfonic acidgroups and the aforementioned aromatic dihalide compounds containingsulfonic acid group are used in a molar ratio of preferably from 0:10 to9:1, more preferably from 0:10 to 8:2, still more preferably from 0:10to 7:3, and the most preferably from 0:10 to 6:4. It is not desirablethat the aromatic dihalide compound containing sulfonic acid group isless than 1, because the proton conductivity becomes lowered.

In the case where the sulfonic acid groups are incorporated bysulfonation after the prepolymer is synthesized, the aromatic dihalidecompound containing sulfonic acid group is not required to be used. Theprepolymer can be also synthesized from the aforementioned aromaticdihalide compound having no sulfonic acid groups and a dihydric phenolhaving no electron-withdrawing group bonded to the aromatic ring.

In the third aspect, the hydrophilic segment prepolymer can besynthesized by the method that is used for synthesizing polyaryletherthrough known nucleophilic substitution reaction. For example, asdisclosed in “Shin Kobunshi Jikkengaku 3, Kobunshi No Gosei•Hanno (2),Shukugokei Kobunshi No Gosei”, edited by The Society of Polymer Science,Japan, published by Kyoritsu Shuppan Co., Ltd., Tokyo, 1996; R. N.Johnson et al., J. Polym. Sci., A-1, Vol. 5, p. 2375 (1967); andJapanese Patent Application Publication No. S46-21458, the hydrophilicsegment prepolymer can be synthesized by reacting a di-alkali metal saltof a dihydric phenol and the aforementioned aromatic dihalide compound.For example,

(1) the aromatic dihalide compound, a dihydric phenol, and an alkalimetal carbonate are mixed with a hydrocarbon solvent so as toazeotropically remove the generating water and a synthesis solvent;

(2) the resulting reaction mixture is heated while stirring so as toremove the generating water while the hydrocarbon solvent is refluxed;and

(3) after the generating water is removed, while or after thehydrocarbon solvent is removed, the reaction mixture is further heatedand stirred so as to obtain the hydrophilic segment prepolymer. In theabove procedure, the step (2) of refluxing the hydrocarbon solvent iscarried out at a temperature of from 100° C. to 170° C. and for from 1hour to 48 hours. The step (3) of further stirring and heating iscarried out at a temperature of from 140° C. to 220° C. for from 0.5hour to 72 hours. The concentration of the resulting prepolymer in thesynthesis solvent is preferably from 0.05 to 0.4 in terms of the weightof the resulting prepolymer with respect to the total weight of theprepolymer and synthesis solvent set equal to 1, more preferably from0.07 to 0.35, particularly preferably from 0.08 to 0.32, and the mostpreferably from 0.1 to 0.3. It is undesirable that the weight of theresulting prepolymer is smaller than 0.05, because a large amount of thesolvent is required. It is also undesirable that the weight is largerthan 0.4, because the objective reaction is not easy to proceed.

In the third aspect, as the alkali metal carbonate that is used tochange the hydroxyl group of the dihydric phenol into an alkali metalsalt, there may be mentioned potassium carbonate, sodium carbonate,lithium carbonate or the like. Potassium carbonate is preferable,because the resulting potassium salt can be used as it is withoutfurther substitution. The charged amounts of the dihydric phenol andaromatic dihalide compound are required to be adjusted, so as to allowthe following block copolymerization to proceed, in such a manner thatterminal hydroxyl groups are formed. The molar ratio of the dihydricphenol to aromatic dihalide compound is in the range of preferably from1.07:1 to 1.001:1, more preferably from 1.05:1 to 1.0015:1, andparticularly preferably from 1.03:1 to 1.002:1. It is undesirable thatthe ratio of the dihydric phenol to the aromatic dihalide compoundbecomes larger than 1.07, because the mechanical strength of the finalproduct block copolymer becomes lowered. It is also undesirable that theratio becomes smaller than 1.001, because the block copolymerizationbecomes difficult to proceed.

In the third aspect, the solvent used for the synthesis has a capabilityof dissolving the resulting prepolymer and may include, for example, apolar solvent such as dimethylsulfoxide, sulfolane,N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone,N,N-dimethylformamide, N,N-dimethylacetoamide and diphenylsulfone.Further, as the hydrocarbon solvent that is admixed so as toazeotropically remove the generating water, there may be mentioned, forexample, an aromatic hydrocarbon solvent such as benzene, toluene andxylene, and an aliphatic hydrocarbon solvent such as hexane,cyclohexane, octane, nonane and decane.

In the case where a unsulfonated prepolymer of the hydrophilic segmentprepolymer is synthesized from an aromatic dihalide compound that has nosulfonic acid groups and a dihydric phenol, the hydrophilic segmentprepolymer can be obtained by sulfonation describing below. In thiscase, the sulfonic acid groups of the resulting sulfonated hydrophilicsegment prepolymer are required to be converted to potassium salt formafter the prepolymer is separated from sulfuric acid solution. There isnot any particular limitation on the conversion method, but theconversion can be completed by immersing the separated sulfonatedprepolymer in a potassium hydroxide or potassium carbonate aqueoussolution from 0.1 wt % to 20 wt % at from 10° C. to 80° C. for from 0.5minute to 48 hours.

The hydrophilic segment prepolymer of the third aspect is composed of astructure represented by the following formula (17) in which thesulfonic acid groups are directly bonded to the aromatic rings.

wherein D² is SO₂ or CO; Ar² is a divalent aromatic residue; and a is aninteger of from 20 to 1,000.

More preferably, the hydrophilic segment prepolymer of the third aspecthas a structure represented by the following formula (18). Particularlypreferably, the structure is polyarylethersulfone that is given byselecting SO₂ for D².

wherein, D² is SO₂ or CO; Ar₂ is a divalent aromatic residue; a is aninteger of from 20 to 1,000; p and q each are independently an integerof 0, 1, or 2; and p+q≧1.

In the third aspect, the hydrophilic segment prepolymer whose sulfonicacid groups are in the form of potassium salt, has a solution viscosity(reduced viscosity) of preferably from 0.3 dl/g to 2.5 dl/g, morepreferably from 0.35 dl/g to 2.3 dl/g, and particularly preferably from0.4 dl/g to 2.0 dl/g at a concentration of 0.5 g/dl and at 25° C. It isundesirable that the solution viscosity is smaller than 0.3 dl/g,because the properties of the resulting block copolymer become lowered.It is also undesirable that the solution viscosity is larger than 2.5dl/g, because the following block copolymerization becomes possiblydifficult.

The hydrophobic segment prepolymer used in the third aspect can besynthesized by the same method as the method of the aforementionedhydrophilic segment prepolymer. As the dihydric phenol and aromaticdihalide compound that are used for the synthesis of the hydrophobicsegment prepolymer, there may be mentioned specifically the dihydricphenols described above and the aromatic dihalide compounds that containno sulfonic acid groups. There is not any particular limitation on thedihydric phenol that is used for the hydrophobic segment synthesis aslong as the resulting block copolymer is not further sulfonated after itis synthesized. However, it is desirable that all of the aromatic ringsof the dihydric phenol are bonded to electron-withdrawing groups,considering the ease of block copolymer synthesis or consideringpossible sulfonation of the hydrophilic segment after the blockcopolymer is synthesized. Such a dihydric phenol may includebis(4-hydroxyphenyl)sulfone and bis(4-hydroxyphenyl)ketone. As thearomatic dihalide compound, there may be mentionedbis(4-chlorophenyl)sulfone and bis(4-fluorophenyl)sulfone, consideringthe ease of the prepolymer synthesis. The molar ratio of the dihydricphenol to aromatic dihalide compound is in the range of preferably from1.05:1 to 1:1.05, more preferably from 1.04:1 to 1:1.04, andparticularly preferably from 1.03:1 to 1:1.03. It is undesirable thatthe ratio of the dihydric phenol to aromatic dihalide compound is out ofthe range of from 1.05:1 to 1:1.05, because the strength of theresulting electrolyte membrane becomes lowered. Note that, there is notany particular limitation of the terminal group structure of thehydrophobic segment, because the block copolymerization is carried outthrough ether exchange reaction using the hydrophilic segment prepolymerthat has terminal hydroxyl groups. In this way, as the hydrophobicsegment prepolymer, a commercially available polymer can be also used.For example, a commercially available polyethersulfone composed of thestructural unit represented by the following formula (19) can be used.

In the third aspect, the hydrophobic segment prepolymer has a solutionviscosity (reduced viscosity) of preferably from 0.3 dl/g to 2.5 dl/g,more preferably from 0.35 dl/g to 2.3 dl/g, and particularly preferablyfrom 0.4 dl/g to 2.0 dl/g at a concentration of 0.5 g/dl and at atemperature of 25° C. It is undesirable that the solution viscosity issmaller than 0.3 dl/g, because the properties of the resulting blockcopolymer become lowered. It is also undesirable that the solutionviscosity is larger than 2.5 dl/g, because the following blockcopolymerization becomes possibly difficult.

In the third aspect, the hydrophilic segment prepolymer and hydrophobicsegment prepolymer can be block copolymerized by using the methoddescribed in Japanese Patent Laid-Open Publication No. 2003-206354. Forexample, the hydrophilic segment prepolymer whose sulfonic acid groupsand terminal hydroxyl groups are in the form of potassium salt and thehydrophobic segment prepolymer can be mixed and reacted in a solution ata temperature of from 120° C. to 200° C., preferably from 130° C. to195° C., and more preferably from 140° C. to 190° C. It is undesirablethat the temperature is lower than 120° C., because the reaction becomesdifficult to proceed. It is also undesirable that the temperature ishigher than 200° C., because the ether exchange reaction progresses toomuch and a random copolymer forms. The reaction time is in the range offrom 15 minutes to 48 hours. It is undesirable that the reaction time isshorter than 15 minutes, because the reaction becomes insufficient. Itis also undesirable that the reaction time becomes longer than 48 hours,because the ether exchange reaction progresses too much and a randomcopolymer forms.

In the block copolymer synthesis, the solutions of both components maybe mixed without further treatments after the solutions are synthesized.Further, both or either component may be used after it is separated anddissolved in a solvent again. Still further, one component in a solidform may be dissolved in the solution of the other component.

Note that, in the case where the separated hydrophilic segmentprepolymer is dissolved again, the hydroxyl groups are required to beconverted into potassium salt form. This conversion can be completed byheating the prepolymer with potassium carbonate in the synthesis solventand hydrocarbon solvent that are used in the prepolymer synthesis; andremoving the generating water. The resulting solution can be used forthe block copolymerization without further treatment. In the case wherethe sulfonic acid groups are in the form of sodium salt, the hydrophilicsegment prepolymer can be used for the block copolymerization after thesulfonic acid groups of the prepolymer are converted to potassium saltform. For example, the hydrophilic segment prepolymer in the form ofsodium salt is immersed in a hydrochloric acid or sulfuric acid aqueoussolution so as to convert the sulfonic acid groups into an acid form,and then the prepolymer is immersed in a potassium hydroxide aqueoussolution 0.1 wt % to 20 wt % at 10° C. to 80° C. for 0.5 minute to 48hours so as to complete the conversion. From the hydrophilic segmentprepolymer that is converted to potassium salt form in this way, asolution used for the block copolymerization can be prepared in the sameprocess as that used in the aforementioned conversion of the hydroxylgroup into potassium salt form.

In the third aspect, the solvent used for the block copolymer synthesishas a capability of dissolving both components, and may include, forexample, a polar solvent such as dimethylsulfoxide, sulfolane,N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone,N,N-dimethylformamide, N,N-dimethylacetoamide and diphenylsulfone, whichis used as the synthesis solvent for both prepolymers. In addition, thehydrocarbon solvent used for the synthesis of both prepolymers may beincluded. The polymer concentration to the polar solvent in the blockcopolymerization is in the range of preferably from 0.05 to 0.4 withrespect to the total of the prepolymers and polar solvent set equal to1, more preferably from 0.07 to 0.35, particularly preferably from 0.08to 0.3. It is undesirable that the concentration of the prepolymers issmaller than 0.05 and larger than 0.4, because the blockcopolymerization becomes difficult to proceed.

In the third aspect, the resulting sulfonated polyarylether blockcopolymer may be subjected to an additional sulfonation. The additionalsulfonation can be completed by dissolving the sulfonated polyaryletherblock copolymer at a concentration of from 2 wt % to 30 wt %, preferablyfrom 3 wt % to 25 wt %, and more preferably from 4 wt % to 20 wt % in aconcentrated sulfuric acid from 80% to 98%, and reacting at atemperature of from 10° C. to 65° C. for 0.1 hour to 168 hours. Notthat, in this case, in order to selectively additionally sulfonate thehydrophilic segment, it is required that no electron-withdrawing groupssuch as sulfone, ketone, nitrile and nitro groups are bonded to thearomatic rings of the hydrophilic segment, and that the aforementionedelectron-withdrawing groups be bonded to almost all of the aromaticrings of the hydrophobic segment. It is undesirable that theconcentration of the sulfonated polyarylether block copolymer is higherthan 30 wt %, because the additional sulfonation becomes difficult. Itis also undesirable, from the viewpoint of cost, that the concentrationis lower than 2 wt %, because the effect on the sulfonation is notchanged. It is undesirable that the concentration of the concentratedsulfuric acid is lower than 80%, because sulfonation sometimes does notproceed. It is also undesirable that the concentration is higher than98%, because even the hydrophobic segment composed of aromatic ringshaving electron-withdrawing groups bonded thereto is possiblysulfonated.

The sulfonated polyarylether block copolymer synthesized by theproduction method according to the third aspect has a hydrophilicsegment weight fraction of preferably from 0.1 to 0.8, more preferablyfrom 0.2 to 0.7, and particularly preferably from 0.25 to 0.65. It isundesirable that the weight fraction is smaller than 0.1, because theproton conductivity becomes lowered. It is also undesirable that theweight fraction is larger than 0.8, because the block copolymer becomeswater-soluble.

The sulfonated polyarylether block copolymer synthesized by theproduction method according to the third aspect has an ion-exchangecapacity of preferably from 0.5 mmol/g to 3.0 mmol/g, more preferablyfrom 0.6 mmol/g to 2.9 mmol/g, and still more preferably from 0.7 mmol/gto 2.8 mmol/g. It is undesirable that the ion-exchange capacity of theblock copolymer is lower than 0.5 mmol/g, because the protonconductivity is lowered. It is also undesirable that the ion-exchangecapacity is larger than 3.0 mmol/g, because the sulfonated blockcopolymer becomes water-soluble or the membrane strength on waterabsorption is largely lowered.

In the third aspect, there is not any particular limitation on theion-exchange capacity of the hydrophilic segment of the sulfonatedpolyarylether block copolymer, but the ion-exchange capacity IEC_(a) ofthe hydrophilic segment solely is preferably 3.6 mmol/g or more from theviewpoint of proton conductivity, more preferably 3.7 mmol/g or more,and particularly preferably 3.8 mmol/g or more.

In the third aspect, the sulfonated polyarylether block copolymer has aproton conductivity, at 70° C. and a relative humidity of 90%, ofpreferably 1×10⁻² S/cm or more, and particularly preferably 1.5×10⁻²S/cm or more. It is undesirable that the proton conductivity is lowerthan 1×10⁻² S/cm, because power generation performance is lowered.Further, at 70° C. and a relative humidity of 30%, the protonconductivity is preferably 1×10⁻³ S/cm or more, more preferably 2.5×10⁻³S/cm or more, and particularly preferably 3.0×10⁻³ S/cm or more. It isundesirable that the proton conductivity at 70° C. and a relativehumidity of 30% is lower than 1×10⁻³ S/cm, because power generationperformance is lowered.

There is not any particular limitation on the method of forming thesulfonated polyarylether block copolymer obtained as described aboveinto a polymer electrolyte membrane. For example, the sulfonatedpolyaryl block copolymer is dissolved in a solvent; the resultingsolution is cast on a support and heated to remove the solvent byevaporation to obtain a membrane. The solvent is not required to befully removed from the membrane on the support, but the membrane ispeeled off from the support after a self-supporting membrane is formed,and then the solvent may be removed by heating. For example, the blockcopolymer is dissolved in a polar solvent such as dimethylsulfoxide,sulfolane, N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone,N,N-dimethylformamide, N,N-dimethylacetoamide and diphenylsulfone; afterthe resulting solution is cast on a support, the solution is dried at80° C. to 250° C. for 0.5 minute to 48 hours to remove the polar solventby evaporation, whereby a membrane can be obtained. Alternatively, whena self-supporting membrane is developed, the membrane may be peeled offand further dried at 80° C. to 250° C. for 0.5 minute to 48 hours toremove the polar solvent by evaporation. It is undesirable that thedrying temperature is lower than 80° C., because the membrane is notfully dried. Above 250° C. is also undesirable, because the membrane ispossibly decomposed.

The polymer electrolyte membrane has a thickness of from 5 to 200 μm,and preferably from 10 μm to 150 μm. The thickness less than 5 μm isundesirable, because the membrane becomes difficult to handle. Thethickness more than 200 μm is also undesirable, because the powergeneration efficiency of fuel cells lowers.

The polymer electrolyte membrane, if necessary, may be allowed to havesulfonic acid groups that are partly converted to metal salts as long asthe properties of the present invention are not impaired. Further, thepolymer electrolyte membrane may be reinforced with fibers, porousmembranes, and others. Still further, if necessary, can be blended aninorganic acid such as phosphoric acid, hypophosphorous acid andsulfuric acid, or their salts, C₁₋₁₄ perfluoroalkylsulfonic acids ortheir salts, C₁₋₁₄ perfluoroalkylcarboxylic acids or their salts, aninorganic material such as platinum, silica gel, silica and zeolite, andother polymers.

There is not any particular limitation on the production method of afuel cell that uses the polymer electrolyte membrane, but there may bementioned the same production method as that used to produce the fuelcell using the polymer electrolyte membrane of the first aspect.

Thus produced fuel cell can be used in a variety of configurations usingas a fuel, hydrogen gas, reformed hydrogen gas, alcohol, ether, or thelike.

Next, the forth aspect of the present invention will be explained. Theproduction method of the polymer electrolyte membrane of the forthaspect is suitably applied to the production of the polymer electrolytemembranes of the first aspect and second aspect.

The phosphate ester used in the forth aspect is the phosphate esterrepresented by the following formula (11) and may include, for example,monocaproyl phosphate, monooctyl phosphate, monocapryl phosphate,monolauryl phosphate, monomyristyl phosphate, monocetyl phosphate,monostearyl phosphate, tetraethyleneglycol mononeopentylethermonophosphate, triethyleneglycol monotridecylether monophosphate,tetraethyleneglycol monolaurylether monophosphate, diethyleneglycolmonostearylether monophosphate, dicaproyl phosphate, dioctyl phosphate,dicapryl phosphate, dilauryl phosphate, dimyristyl phosphate, dicetylphosphate, distearyl phosphate, tetraethyleneglycol mononeopentyletherdiphosphate, triethyleneglycol monotridecylether diphosphate,tetraethyleneglycol monolaurylether diphosphate, diethyleneglycolmonostearylether diphosphate and the like.

wherein R¹ is a hydrogen atom, an alkyl group having 6 to 18 carbonatoms, or a group represented by the following formula (13); and R² isan alkyl group having 6 to 18 carbon atoms or a group represented by thefollowing formula (13),

[Formula 16]

R³—(OC₂H₄)_(m)—  (13)

wherein R³ is an alkyl group having 5 to 18 carbon atoms; and m is aninteger of from 2 to 30,

The salt of amine and phosphate ester used in the forth aspect is thesalt obtained from the foregoing phosphate ester and an aminerepresented by the following formula (12).

wherein R₄ to R₆ each are a hydrogen atom, a hydroxyethyl group, or analkyl group having 1 to 12 carbon atoms.

The amine represented by the above formula (12) may include, forexample, ammonia, monomethylamine, monoethylamine, monopropylamine,monobutylamine, monohexylamine, monooctylamine, monolaurylamine,dimethylamine, diethylamine, dipropylamine, dibutylamine, dihexylamine,dioctylamine, dilaurylamine, trimethylamine, triethylamine,tripropylamine, tributylamine, trihexylamine, trioctylamine,trilaurylamine, monoethanolamine, diethanolamine, triethanolamine andthe like.

As the aforementioned alkylphosphate, a commercially available productmay be used, which includes, for example, “SEPARL328”, “SEPARL365”,“SEPARL380”, “SEPARL440”, “SEPARL441”, “SEPARL517”, “SEPARL521” (tradename), manufactured by Chukyo Yushi Co., Ltd., and the like.

The phosphate ester and/or salt between phosphate ester and amine are/isadmixed in an amount of from 0.0005 to 2 parts by weight, preferablyfrom 0.001 to 2 parts by weight, more preferably from 0.002 to 1.5 partsby weight, and still more preferably from 0.003 to 1.0 part by weightwith respect to 100 parts by weight of the polymer electrolyte. When theamount added is less than 0.0005 part by weight, no effect is obtained.On the other hand, when the amount added is more than 2 parts by weight,the effect is not changed.

The polymer electrolyte used in the forth aspect has strong acid groupssuch as sulfonic acid groups and phosphoric acid groups, or superstrongacid groups such as sulfonic acid groups bonded to fluoroalkyl orfluoroaryl groups, and may include an aliphatic hydrocarbon polymer, analiphatic polymer whose hydrogen atoms are substituted for fluorineatoms, an aromatic polymer having aromatic rings in its main chain andthe like.

These polymers may include, for example, a polymer having an aliphaticmain chain such as polystyrene sulfonic acid; polyvinylbenzyl sulfonicacid; styrene-(ethylene-butylene)-styrene triblock copolymer orstyrene-(ethylene-propylene) block copolymer that contains sulfonic acidgroups as described in Japanese Patent Application Laid-Open No.2002-509152 and European Polymer Journal, Vol. 36, p. 61 (2001);styrene-(ethylene-butylene)-styrene triblock copolymer orstyrene-(ethylene-propylene) block copolymer that contains carboxylicacid groups as described in Macromolecules, Vol. 28, p. 8702 (1995) orEuropean Polymer Journal, Vol. 36, p. 61 (2001); a polystyrene polymerhaving strong acid groups such as a polystyrene having phosphonic acidgroups as described in Japanese Patent Laid-Open Publication No.2000-11755; polyacrylic acid; polymethacrylic acid; a vinylpolymerhaving strong acid groups such as polyvinylsulfonic acid; and aperfluoro polymer having superstrong acid groups such as Nafion(registered trademark), Aciplex (registered trademark), and Flemion(registered trademark).

In addition, there may be mentioned an aromatic polymer electrolyte,including polyethersulfone, polysulfone, polyetherketone,polyetheretherketone, polyetherketoneketone, polyimide,polyphenyleneoxide, polyarylene, polyphenylenesulfide and the like thatcontain strong acid groups or superstrong acid groups. For example,there may be mentioned, an aromatic polyethersulfone that containssulfonic acid groups as described in Japanese Patent Laid-OpenPublication No. S61-43630; J. Membr. Sci., Vol. 83, p. 211 (1993); J.Polym. Sci., Part A, Polym. Chem., Vol. 34, p. 2421 (1996); J. Polym.Sci., Part A, Polym. Chem., Vol. 31, p. 853 (1993); U.S. Patent No.2001/0021764A1; and Japanese Patent Laid-Open Publication No.2003-31232; an aromatic polyetherketone that contains sulfonic acidgroups as described in Japanese Patent Laid-Open Publication No.S57-25328; Japanese Patent Laid-Open Publication No. H06-93114; J.Membr. Sci., Vol. 199, p. 167 (2002); J. Membr. Sci., Vol. 173, p. 17(2000); Polymer, Vol. 28, p. 1009 (1987); Solid State Ionics, Vol. 106,p. 219 (1998); Br. Polym. J., Vol. 17, p. 4 (1985); and Polym. Int.,Vol. 50, p. 812 (2001); a polyimide that contains sulfonic acid groupsas described in Polymer Preprint, Japan of The Society of PolymerScience, Japan, Vol. 51, p. 744-746 (2002); a polyphenyleneoxide thatcontains sulfonic acid groups as described in J. Appl. Polym. Sci., Vol.51, p. 1399 (1994); J. Appl. Polym. Sci., Vol. 29, p. 4017 (1984); J.Appl. Polym. Sci., Vol. 29, p. 4029 (1984); and J. Membr. Sci., Vol.146, p. 263 (1998); and a sulfonated polyarylene as described inJapanese Patent Laid-Open Publication No. 2005-112985.

These polymer electrolytes may be a copolymer, a random copolymer, ablock copolymer, or a graft copolymer. In particular, from the viewpointof proton conductivity, these polymer electrolytes desirably have ablock copolymer structure. Among these polymer electrolytes, an aromaticpolymer electrolyte is preferable from the viewpoint of heat-resistanceand cost. Further, from the viewpoint of ion conductivity, an aromaticpolymer electrolyte that contains sulfonic acid groups is preferable.Further, within the range of ion-exchange capacity describing below,there may be mentioned a polymer electrolyte composed of a polymer-blendcomposition that contains a polymer that has no strong acid groups orsuperstrong acid groups and the aforementioned polymer that has strongacid groups or superstrong acid groups. As the polymer having no strongacid groups or superstrong acid groups, there may be mentionedspecifically, the aforementioned polymer having no strong acid groups orsuperstrong acid groups. A combination of a polymer that has strong acidgroups or superstrong acid groups and a polymer that has no strong acidgroups or superstrong acid groups and has the same structural unit asthat of the former polymer is preferable.

These polymer electrolytes have an ion-exchange capacity of preferablyfrom 0.2 mmol/g to 4 mmol/g, and more preferably from 0.3 mmol/g to 3.5mmol/g. It is undesirable that the ion-exchange capacity is smaller than0.2 mmol/g, because proton conductivity becomes lowered, for example,for fuel cell applications. It is also undesirable that the ion-exchangecapacity is larger than 4 mmol/g, because the polymer electrolytesbecome water-soluble.

As the polymer electrolyte used in the fourth aspect, the polymerelectrolyte of the first aspect or the polymer electrolyte compositionof the second aspect is particularly preferable.

There is not any particular limitation on the solvent used in the forthaspect as long as the solvent can dissolve the objective polymerelectrolyte, and phosphate ester or a salt of phosphate ester and amine.For example, in the case of aromatic polymer electrolytes, there may bementioned a polar solvent such as dimethylsulfoxide, sulfolane,N-methyl-2-pyrolidone, 1,3-dimethyl-2-imidazolidinone,N,N-dimethylformamide, N,N-dimethylacetoamide, diphenylsulfone, phenol,m-cresol, and p-chlorophenol.

In the production method of the polymer electrolyte membrane accordingto the forth aspect, when a polymer electrolyte solution is heated toevaporate the solvent until a self-supporting membrane is obtained afterthe polymer electrolyte solution is cast on a support, there is not anyparticular limitation on the drying condition as long as no foamdevelops. Generally, the solution is dried in the range of from roomtemperature to the temperature higher by 20° C. than the boiling pointof the solvent, for 0.2 minute to 72 hours. In particular, vacuum dryingor hot-air drying is desirable because drying speed can be enhanced. Inaddition, after the self-supporting membrane is peeled off from thesupport, if necessary, the membrane may be further dried at atemperature of from room temperature to 280° C. for 0.2 minute to 72hours.

As the material for the support used in the fourth aspect, there may bementioned rubber, plastics, metals such as stainless steel, ceramics,glass, and others. From the viewpoint of durability, metals such asstainless steel, ceramics, glass and others are preferable. The shape ofthe support may be a plate, a belt, a roll and the like. In particular,a belt or roll shaped support is desirable, because the membrane can beproduced continuously.

In the forth aspect, a polymer electrolyte membrane is prepared asfollows: a polymer electrolyte solution is cast on a support and heatedto evaporate the solvent until a self-supporting membrane is formed; theresulting self-supporting membrane is peeled off from the support; andif necessary, the membrane is further dried. Thus prepared polymerelectrolyte membrane may be used as it is, but the membrane is desirablysubjected to an acid aqueous solution treatment depending onapplications such as fuel cells. Further, if necessary, the membrane maybe subjected to the acid aqueous solution treatment after the membraneis subjected to an alkali aqueous solution treatment. In each treatment,the membrane may be immersed in each aqueous solution for from 0.05minute to 600 minutes in batch-wise or continuously. Short immersion isundesirable, because the treatment becomes insufficient. Long immersionis also undesirable, because no further change in the effect can beexpected. For example, as described above, after each treatment, ifnecessary, water-washing process and/or drying process may beincorporated as follows: (1) alkali treatment→(2) water washing→(3) acidtreatment→(4) water washing→(5) drying.

In the case where phosphate ester or the salt of phosphate ester andamine is used in an amount more than 0.5 wt %, it is desirable that theadded phosphate ester or the salt of phosphate ester and amine isremoved in the alkali aqueous solution treatment followed by the acidaqueous solution treatment. Eventually, the phosphate ester or the saltof phosphate ester and amine remained in the polymer electrolytemembrane is reduced below preferably 0.5 wt % or less, and morepreferably 0.4 wt % or less. It is undesirable that the phosphate esteror the salt of phosphate ester and amine remains too much, because thedurability or power generation performance is possibly lowered.

EXAMPLES

The present invention will be further described in detail with referenceto the following examples and comparative examples. The measured valuesin the examples and comparative examples were obtained by the followingmethods.

1) Solution Viscosity η sp/c (Reduced Viscosity) Measurement

A test specimen was dissolved in N-methyl-2-pyrolidone (containing 50mmol/L of LiCl) in a solution with a concentration of 0.5 g/dl. Theresulting solution was measured with an Ubbelode viscometer at 25° C.,and the viscosity was calculated by the following equation (1):

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu} 7} \right\rbrack & \; \\{\eta_{{sp}/c} = {\frac{t_{s} - t_{0}}{t_{0}} \cdot \frac{1}{c}}} & (1)\end{matrix}$

wherein, t_(s) is the time measured for the solution; t₀ is the timemeasured for the solvent; and c is the concentration of the solution.

2) Proton Conductivity Measurement

In a thermo-hygrostat, a test specimen membrane (5 mm wide×20 mm long)was sandwiched between a Teflon (registered trademark) plate that had aslit of 1.9 mm wide and 10 mm long and platinum wires on both sides ofthe slit (at a spacing of 2 mm) and another Teflon (registeredtrademark) flat plate in such a manner that the longitudinal directionof the membrane was directed at an angle of 90 degree to the platinumwires. At a temperature of 50° C. or 70° C. with changing the relativehumidity, the proton conductivity of the membrane was evaluated by thecomplex impedance measurement using LCR HiTESTER 3532 manufactured byHIOKI E. E. Corporation.

3) Ion-Exchange Capacity Measurement

A test specimen was put in a sodium hydroxide aqueous solution with aknown concentration; after stirring at room temperature for 16 hours,the solution was filtered off; the resulting filtrate was titrated witha 0.01 N hydrochloric acid solution so as to measure the consumed amountof sodium hydroxide; and then the ion-exchange capacity was calculated.

4) Transmission Electron Microscope Observation

A thin test specimen was prepared by cutting the membrane in thethickness direction, and was observed with JEM-200CX manufactured byJEOL Ltd. at a magnification of 90,000.

5) H-NMR Measurement

AL-300 and EX-400WB manufactured by JEOD Ltd., and a d-DMSO as asolvent, were used for measurement.

6) Evaluation of Properties on Water-Absorption

A TENSILON tensile tester was used. The measurement conditions are asfollows. Note that, the properties on water-absorption were measured bydrawing a test specimen in water.

Test specimen size: 5 mm of width, 30 mm of length between chucks

Tensile speed: 30 mm/min

Test temperature: 23° C.

7) Evaluation of Peeling Properties

On a stainless steel plate (SUS304, 40 mm×150 mm), a sample solution wascast and dried under predetermined conditions. Peeling properties wereevaluated by the condition of the resulting self-supporting film when itwas peeled off from the stainless steel plate.

Synthesis Example 1 Synthesis of3,3′-Disulfo-4,4′-Difluorodiphenylsulfone Sodium Salt

In a flask, 120 g of bis(4-fluorophenyl)sulfone and 250 g of 30% fumingsulfuric acid were charged and heated at 110° C. for 6 hours whilestirring. After the resulting solution was gradually added to ice water,sodium chloride was added so as to deposit a solid product. The solidproduct was dissolved again in water. After the resulting solution wasneutralized with NaOH, sodium chloride was added so as to deposit asolid product again. The solid product was recrystallized twice with a2-propanol/water (7/3) mixture and dried to obtain a white solidproduct. Thus obtained white solid product provided H-NMR signals atfrom 7.4 ppm to 7.5 ppm, from 7.9 ppm to 8.0 ppm, and from 8.1 ppm to8.2 ppm with the integrated intensities of 1:1:1. The white solidproduct was identified to be 3,3′-disulfo-4,4′-difluorodiphenylsulfonesodium salt.

Example 1 Polymerization of Polyethersulfone Block Copolymer PB1

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 10.17 g ofbis(4-fluorophenyl)sulfone, 27.5 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17.5 g of potassium carbonate were charged; after 210ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under nitrogen flow. The temperature waselevated to 175° C. while removing the generating water together withtoluene, and then the reaction mixture was stirred at that temperaturefor 16 hours to obtain a hydrophilic prepolymer HP1 solution.Separately, 81.47 g of bis(4-fluorophenyl)sulfone, 78.99 g ofbis(4-hydroxyphenyl) sulfone, and 52 g of potassium carbonate werecharged; after 600 ml of dimethylsulfoxide and 50 ml of toluene wereadded, the reaction mixture was heated and stirred under a nitrogen gasatmosphere. The temperature was elevated to 175° C. while removing thegenerating water together with toluene, and then the reaction mixturewas stirred at that temperature for 16 hours to obtain a hydrophobicprepolymer SP1 solution. The SP1 solution was added to the HP1 solution,and then the mixed solution was stirred at 170° C. for 1.5 hours. Thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in hot water twice, and in methanol once, whereby a blockcopolymer PB1 was obtained. The solution viscosity η sp/c of theobtained polymer was 0.61 dl/g. The ion-exchange capacity was 0.59mmol/g.

Sulfonation of Block Copolymer PB1 (SPB1 Synthesis)

20 g of the block copolymer PB1 were dissolved in 180 g of 98% sulfuricacid, and the resulting solution was stirred at room temperature for 24hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidwas washed in water 5 times to obtain a polymer SPB1. The ion-exchangecapacity of the polymer was 1.46 mmol/g. The hydrophilic segment, whichis water-soluble when it acts solely, is removed by water washing aftersulfonation if the product is a blend as mentioned in Referenced Example2, and the ion-exchange capacity is largely lowered. However, theion-exchange capacity of SPB1 gave almost the same value as thecalculated value, 1.49 mmol/g obtained by assuming that one sulfonicacid group was incorporated into each of the biphenol residue aromaticrings of PB1 before sulfonation. This shows that the hydrophilic segmentand hydrophobic segment are bonded together. In addition, in themembrane prepared by the method described below, a phase-separationstructure having an average domain distance of 60 nm was found by TEMobservation. Hence, SPB1 was identified to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions. Therefore, only the hydrophilic segment is considered tohave sulfonic acid groups. Based on the SPB1 composition obtained by theH-NMR measurement, the hydrophilic segment weight fraction after thesulfonic acid groups were converted to SO₃H was 0.31. The ion-exchangecapacity of the hydrophilic segment of SPB1 was 4.73 mmol/g.

SPB1 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 130° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour to obtain a 50 μm thickmembrane. The resulting membrane was immersed in a 1 N sodium hydroxideaqueous solution at room temperature for 2 hours, water-washed, and thenimmersed in a 1 N sulfuric acid aqueous solution for 4 hours. Themembrane was water-washed 3 times. After the washing water was confirmedto be neutral, the proton conductivity was measured at 50° C. and 70° C.with changing the relative humidity. The results are shown in Table 1and FIG. 1. As compared with Comparative Example 1 described below, theproton conductivity was remarkably improved even at the sameion-exchange capacity.

Example 2 Preparation of Polyethersulfone Block Copolymer PB2

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 12.71 g ofbis(4-fluorophenyl)sulfone, 22.91 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17 g of potassium carbonate were charged; after 200ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 175° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to obtain a hydrophilic prepolymer HP2solution. Separately, 100 g of “SUMIKAEXCEL 7600P” (manufactured bySumitomo Chemical Co., Ltd.) having the repeating unit represented bythe following formula,

were dissolved in 310 ml of dimethylsulfoxide; and then 50 ml of toluenewere added so as to obtain a hydrophobic prepolymer SP2 solution thatwas azeotropically dehydrated. The SP2 solution was added to the HP2solution, and then the mixed solution was stirred at 170° C. for 1.5hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB2 was obtained. The solution viscosity η sp/c of theobtained polymer was 0.65 dl/g. The ion-exchange capacity was 0.79mmol/g.

Sulfonation of Block Copolymer PB2 (SPB2 Synthesis)

In 180 g of 98% sulfuric acid, 20 g of the block copolymer PB2 weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a polymer SPB2. Theion-exchange capacity of the polymer was 1.80 mmol/g. The hydrophilicsegment, which is water-soluble when it acts solely, is removed by waterwashing, and the ion-exchange capacity is largely lowered. However, theion-exchange capacity of SPB2 gave almost the same value as thecalculated value, 1.84 mmol/g obtained by assuming that one sulfonicacid group was incorporated into each of the biphenol residue aromaticrings of PB2 before sulfonation. This shows that the hydrophilic segmentand hydrophobic segment are bonded together. In addition, in themembrane prepared by the method described below, a phase-separationstructure having an average domain distance of 31 nm was found by TEMobservation. Hence, SPB2 was identified to be a block copolymer. Asshown in Referenced Example 1, the hydrophobic segment is not sulfonatedunder the foregoing sulfonation conditions. Therefore, only thehydrophilic segment is considered to have sulfonic acid groups. Based onthe SPB2 composition obtained by the H-NMR measurement, the hydrophilicsegment weight fraction after the sulfonic acid groups were converted toSO₃H was 0.39. The ion-exchange capacity of the hydrophilic segment ofSPB2 was 4.60 mmol/g.

SPB2 was formed in the same manner as in Example 1 to obtain a 48 μmmembrane. The proton conductivity of the membrane was measured. Theresults are shown in Table 1 and FIG. 1. As compared with ComparativeExample 2 described below, the proton conductivity was remarkablyimproved even at almost the same ion-exchange capacity.

Comparative Example 1

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 42.7 g ofbis(4-chlorophenyl)sulfone, 28.2 g of 4,4′-biphenol, and 27.2 g ofpotassium carbonate were charged; after 240 ml of dimethylsulfoxide and30 ml of toluene were added, the reaction mixture was heated and stirredunder nitrogen flow. The temperature was elevated to 180° C. whileremoving the generating water together with toluene, and then thereaction mixture was stirred at that temperature for 4 hours to obtain ahydrophilic segment prepolymer HP1′ solution. Separately, a solution wasprepared by dissolving 115 g of “SUMIKAEXCEL 7600P” in 345 ml ofdimethylsulfoxide. The solution was poured into the HP1′ solution, andthe mixed solution was stirred at 170° C. for 1.5 hours. After that, thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. The solid product was washedin hot water twice and in methanol once to obtain a block copolymerPB1′. The solution viscosity η sp/c of the obtained polymer was 0.71dl/g.

In 90 g of 98% sulfuric acid, 10 g of PB1′ were dissolved, and theresulting solution was stirred at room temperature for 24 hours. Thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in hot water twice and in methanol once to obtain asulfonated block copolymer SPB1′. The ion-exchange capacity of theobtained polymer was 1.44 mmol/g. This indicates that the hydrophilicsegment was not dissolved and removed during the hot-water washing, andthat the hydrophilic segment and hydrophobic segment were bondedtogether. In addition, in the membrane prepared in the same manner as inExample 1, a phase separation structure was found by TEM observation.This indicates that the obtained polymer SPB1′ was a block copolymer.Based on the SPB1′ composition obtained by the H-NMR measurement, thehydrophilic segment weight fraction after the sulfonic acid groups wereconverted to SO₃H was 0.41. The ion-exchange capacity of the hydrophilicsegment of SPB1′ was 3.51 mmol/g.

SPB1′ was formed into a membrane in the same manner as in Example 1, andthe proton conductivity of the membrane was measured. The results areshown in Table 1 and FIG. 1.

Comparative Example 2

A sulfonated block copolymer SPB2′ was synthesized in the same manner asin Comparative Example 1, except that a block copolymer was synthesizedby using 67 g of “SUMIKAEXCEL 7600P” and 200 ml of dimethylsulfoxidethat was used to prepare the solution thereof. The ion-exchange capacityof the obtained SPB2′ was 1.77 mmol/g. Based on the SPB2′ compositionobtained by the H-NMR measurement, the hydrophilic segment weightfraction after the sulfonic acid groups were converted to SO₃H wascalculated to be 0.5. The ion-exchange capacity of the hydrophilicsegment of SPB2′ was 3.54 mmol/g.

SPB2′ was formed into a membrane in the same manner as in Example 1, andthe proton conductivity of the membrane was measured. The results areshown in Table 1 and FIG. 1.

Referenced Example 1

In 180 g of 98% sulfuric acid, 20 grams of “SUMIKAEXCEL 7600P” weredissolved. The resulting solution was stirred at room temperature for 24hours. The solution was poured into a large amount of water so as todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times. The ion-exchangecapacity of the obtained polymer was not able to be measured, and theincorporation of sulfonic acid groups was not identified.

Referenced Example 2

A hydrophilic prepolymer was synthesized in the same manner as inExample 1, and put in water to obtain a solid product. The solutionviscosity of the obtained polymer was 0.35. 52 g of the polymer wasmixed and dissolved at room temperature in a hydrophobic prepolymersolution synthesized in the same manner as in Example 1. The resultingsolution was poured into water to obtain a solid product that was ablend of the hydrophilic prepolymer and hydrophobic prepolymer. In 180 gof 98% sulfuric acid, 20 g of the blend were dissolved, and theresulting solution was stirred at room temperature for 24 hours. Thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in water 5 times. The ion-exchange capacity of the obtainedpolymer was not able to be measured. In addition, in the H-NMRmeasurement, only signals derived from the hydrophobic prepolymer wereobserved. This means that the hydrophilic prepolymer becamewater-soluble after the sulfonation, and was removed in thewater-washing process.

Example 3 Synthesis of Polyethersulfone Block Copolymer PB3

In a four-neck separable flask equipped with a stirrer, a Dean-Starktrap, a thermometer, and a nitrogen gas inlet, 3.35 g (0.012 mol) ofbis(4-chlorophenyl) sulfone, 18.34 g (0.035 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, 8.79 g (0.047mol) of 4,4′-biphenol, and 8.22 g of potassium carbonate were charged;after 80 g of dimethylsulfoxide and 35 g of toluene were added, thereaction mixture was heated and stirred under nitrogen flow. Thetemperature was elevated to 160° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 8 hours to obtain a hydrophilic segment prepolymer HP3solution. Here, the ratio of 4,4′-biphenol that is a dihydric phenol tothe aromatic dichloride was 1.01:1. In the HP3 solution, the resultingprepolymer concentration with respect to the total amount of theprepolymer and dimethylsulfoxide was 25.2 wt %. Separately, 40.38 g of“SUMIKAEXCEL 7600P” were dissolved in 160 g of dimethylsulfoxide and 80g of toluene, and then the resulting solution was heated and stirredunder nitrogen flow. The temperature was elevated to 185° C. whileremoving the flowing water together with toluene, and then the solutionwas stirred at that temperature for 8 hours to obtain a hydrophobicsegment prepolymer SP3 solution. In the HP3 solution, the concentrationof the prepolymer SP3 with respect to the total amount of the prepolymerSP3 and dimethylsulfoxide was 20.2 wt %. The SP3 solution was added tothe HP3 solution, and then the mixed solution was stirred at 160° C. for2 hours. The solution was poured into a large amount of water to deposita white solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB3 was obtained. The solution viscosity η sp/c of theobtained polymer was 1.05 dl/g. The ion-exchange capacity was 0.92mmol/g.

Sulfonation of Block Copolymer PB3 (SPB3 Synthesis)

In 332 g of 98% sulfuric acid, 25 g of the block copolymer PB3 weredissolved, and the resulting solution was stirred at 40° C. for 48hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidwas washed in water 5 times to obtain a polymer SPB3. The ion-exchangecapacity of the obtained polymer was 2.00 mmol/g. If the polymer is ablend, the hydrophilic polymer that is water-soluble is removed by waterwashing after sulfonation, and the ion-exchange capacity is largelylowered. However, the ion-exchange capacity of SPB3 gave almost the samevalue as the calculated value, 2.08 mmol/g obtained by assuming that onesulfonic acid group was incorporated into each of the biphenol residuearomatic rings of PB3 before sulfonation. This indicates that SPB3 isnot a blend, but the hydrophilic segment and hydrophobic segment arebonded together. In addition, in the membrane prepared by the methoddescribed below, a phase-separation structure having an average domaindistance of 51 nm was shown by TEM observation. Hence, SPB3 wasidentified to be a block copolymer. The hydrophobic segment is notsulfonated under the foregoing sulfonation conditions. Therefore, onlythe hydrophilic segment is considered to have sulfonic acid groups.Based on the SPB3 composition obtained by the H-NMR measurement, thehydrophilic segment weight fraction after the sulfonic acid groups wereconverted to SO₃H was 0.41. The ion-exchange capacity of the hydrophilicsegment of SPB3 was 4.88 mmol/g.

SPB3 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 120° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, and then immersed in a 1 Nsulfuric acid aqueous solution for 4 hours. The membrane waswater-washed 3 times. After the washing water was confirmed to beneutral, the membrane was fixed on a metal frame and dried at 40° C. toobtain a 32 μm thick membrane. The proton conductivity was measured at50° C. and 70° C. with changing the relative humidity. The results areshown in Table 1.

Example 4 Synthesis of Prepolymer HP4

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 8.41 g (0.045 mol) of4,4′-biphenol and 50 g of N,N-dimethylacetoamide were charged. They werestirred at 60° C. with nitrogen gas bubbling and dissolved. After 7.87 gof potassium carbonate and 15 g of toluene were added, they were heatedand stirred at 160° C. under nitrogen flow so as to remove thegenerating water together with toluene. After that, 6.38 g (0.022 mol)of bis(4-chlorophenyl)sulfone, 11.64 g (0.022 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, and 60 g ofN,N-dimethylacetoamide were added. The resulting reaction mixture wasstirred at 160° C. for 16 hours under nitrogen flow to obtain ahydrophilic segment prepolymer HP4 solution. Here, the ratio of4,4′-biphenol that is a dihydric phenol to the aromatic dichloride was1.016:1. In the HP4 solution, the resulting prepolymer concentrationwith respect to the total amount of the prepolymer andN,N-dimethylacetoamide was 18 wt %. After insoluble substance wasfiltered off, the resulting filtrate was poured into a large amount of2-propanol to deposit a white solid product, which was then vacuum-driedat 100° C. and dissolved again in N,N-dimethylacetoamide. The processesof filtration, deposition with 2-propanol, and drying were repeated toobtain a prepolymer HP4 having sulfonic acid groups in potassium form.The η sp/c of HP4 was 0.89 dl/g. The ion-exchange capacity was 1.91mmol/g.

Synthesis of Block Copolymer PB4

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 6.05 g of HP4 and 55 g ofN,N-dimethylacetoamide were charged. They were stirred at 80° C.overnight under nitrogen flow so as to dissolve HP4. After 0.029 g ofpotassium carbonate and 10 g of toluene were added, the solution washeated and stirred at 160° C. under nitrogen flow to remove thegenerating water together with toluene. Separately, 7.36 g of“SUMIKAEXCEL 7600P” were dissolved in 36 g of N,N-dimethylacetoamide bystirring at 80° C. overnight under nitrogen flow, and then the resultingsolution was dehydrated by adding 10 g of toluene so as to obtain ahydrophobic segment prepolymer SP4 solution. The SP4 solution was addedto the HP4 solution, and then the mixed solution was stirred at 160° C.for 2 hours. The solution was filtered to remove the insolublesubstance, and poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in hot water twice, and in methanol once, whereby a blockcopolymer PB4 was obtained. The solution viscosity η sp/c of theobtained polymer was 1.09 dl/g. The ion-exchange capacity was 0.643mmol/g.

Sulfonation of Block Copolymer PB4 (SPB4 Synthesis)

In 63.6 g of 95% sulfuric acid, 7.07 g of the block copolymer PB4 wasdissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a polymer SPB4. Ifthe polymer is a blend, the hydrophilic polymer that is water-soluble isremoved by water washing after sulfonation and the yield is largelylowered. However, the yield of SPB4 was 94% of the calculated valueobtained by assuming that one sulfonic acid group was incorporated intoeach of the biphenol residue aromatic rings of PB4 before sulfonation.This indicates that SPB4 is not a blend, but the hydrophilic segment andhydrophobic segment are bonded together. The ion-exchange capacity was1.72 mmol/g. In addition, in the membrane prepared by the methoddescribed below, a phase-separation structure having an average domaindistance of 58 nm was shown by TEM observation. Hence, SPB4 wasidentified to be a block copolymer. The hydrophobic segment is notsulfonated under the foregoing sulfonation conditions. Therefore, onlythe hydrophilic segment is considered to have sulfonic acid groups.Based on the SPB4 composition obtained by the H-NMR measurement, thehydrophilic segment weight fraction after the sulfonic acid groups wereconverted to SO₃H was 0.46. The ion-exchange capacity of the hydrophilicsegment of SPB4 was 4.20 mmol/g.

SPB4 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 120° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, and then immersed in a 1 Nsulfuric acid aqueous solution for 4 hours. The membrane waswater-washed 3 times. After the washing water was confirmed to beneutral, the membrane was fixed on a metal frame and dried at 40° C. toobtain a 21 μm thick membrane. The proton conductivity was measured at50° C. and 70° C. with changing the relative humidity. The results areshown in Table 1.

Example 5 Synthesis of Polyethersulfone Block Copolymer PB5

In a four-neck separable flask equipped with a stirrer, a Dean-Starktrap, a thermometer, and a nitrogen gas inlet, 25.33 g (0.0484 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, 9.10 g (0.0489mol) of 4,4′-biphenol, and 8.51 g of potassium carbonate were charged;after 122 g of dimethylsulfoxide and 45 g of toluene were added, thereaction mixture was heated and stirred under a nitrogen gas atmosphere.The temperature was elevated to 160° C. while removing the generatingwater together with toluene, and then the reaction mixture was stirredat that temperature for 8 hours to prepare a hydrophilic segmentprepolymer HP5 solution. Here, the ratio of 4,4′-biphenol that is adihydric phenol to the aromatic dichloride was 1.01:1. In the HP5solution, the resulting prepolymer concentration with respect to thetotal amount of the prepolymer and dimethylsulfoxide was 20.2 wt %.Separately, 42.2 g of “SUMIKAEXCEL 7600P” were dissolved in 211 g ofdimethylsulfoxide and 28 g of toluene, and then the resulting solutionwas heated and stirred under nitrogen flow. The temperature was elevatedto 185° C. while removing the flowing water together with toluene, andthen the solution was stirred at that temperature for 8 hours to preparea hydrophobic segment prepolymer SP5 solution. In the SP5 solution, theconcentration of the prepolymer SP5 with respect to the total amount ofthe prepolymer SP5 and dimethylsulfoxide was 20.0 wt %. The SP5 solutionwas added to the HP5 solution, and then the mixed solution was stirredat 160° C. for 2 hours. The solution was poured into a large amount ofwater to deposit a white solid product, which was then filtered off.Thus obtained solid product was washed in hot water twice, and inmethanol once, whereby a block copolymer PB5 was obtained. The solutionviscosity η sp/c of the obtained polymer was 0.84 dl/g. The ion-exchangecapacity was 0.931 mmol/g.

Sulfonation of Block Copolymer PB5 (SPB5 Synthesis)

In 351 g of 98% sulfuric acid, 30 g of the block copolymer PB5 weredissolved, and the resulting solution was stirred at room temperaturefor 72 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a polymer SPB5. Theion-exchange capacity of the polymer was 1.69 mmol/g. If the polymer isa blend, the hydrophilic polymer that is water-soluble is removed bywater washing after sulfonation and the ion-exchange capacity is largelylowered. However, the ion-exchange capacity of SPB5 gave almost the samevalue as the calculated value, 1.69 mmol/g obtained by using thecomposition ratio obtained by the H-NMR measurement for PB5 beforesulfonation, and by assuming that one sulfonic acid group wasincorporated into each of the biphenol residue aromatic rings beforesulfonation. This indicates that SPB5 is not a blend, but thehydrophilic segment and hydrophobic segment are bonded together. Inaddition, in the membrane prepared by the method described below, aphase-separation structure was shown by TEM observation. Hence, SPB5 wasidentified to be a block copolymer. The hydrophobic segment is notsulfonated under the foregoing sulfonation conditions. Therefore, onlythe hydrophilic segment is considered to have sulfonic acid groups.Based on the SPB5 composition obtained by the H-NMR measurement, thehydrophilic segment weight fraction after the sulfonic acid groups wereconverted to SO₃H was 0.30. The ion-exchange capacity of the hydrophilicsegment of SPB5 was 5.63 mmol/g.

SPB5 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 120° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, and then immersed in a 1 Nsulfuric acid aqueous solution for 4 hours. The membrane waswater-washed 3 times. After the washing water was confirmed to beneutral, the membrane was fixed on a metal frame and dried at 40° C. toobtain a 38 μm thick membrane. The proton conductivity was measured at70° C. with changing the relative humidity. The results are shown inTable 1.

TABLE 1 Properties of aromatic block copolymers Proton conductivityIon-exchange capacity 50° C. 70° C. Whole Hydrophilic 40% 60% 90% 30%90% polymer segment RH*¹ RH*¹ RH*¹ RH*¹ RH*¹ (mmol/g) (mmol/g) (S/cm)(S/cm) (S/cm) (S/cm) (S/cm) Example 1 1.46 4.73 6 × 10⁻³ 1.5 × 10⁻² 0.132.3 × 10⁻³ 0.11 Comparative 1.44 3.51 1 × 10⁻³  5 × 10⁻³ 4.8 × 10⁻² 8.7× 10⁻⁴ 8.2 × 10⁻² Example 1 Example 2 1.80 4.60 9 × 10⁻³ 3.2 × 10⁻² 0.184.0 × 10⁻³ 0.20 Comparative 1.77 3.54 3 × 10⁻³ 9.6 × 10⁻³ 0.11 1.9 ×10⁻³ 0.14 Example 2 Example 3 2.00 4.88 1.1 × 10⁻²  0.21 6.6 × 10⁻³ 0.26Example 4 1.72 4.20 8.7 × 10⁻³  0.14 3.7 × 10⁻³ 0.21 Example 5 1.69 5.636.1 × 10⁻³ 0.23 *¹Relative humidity

Example 6

On both sides of the membrane obtained in Example 2, gas diffusionelectrodes EC20-10-10 (1.0 mg/cm² of Pt content) manufacture byElectroChem Inc. were attached by pressing at 130° C. for 5 minutes toprepare a membrane/electrode assembly (hereinafter, referred to as“MEA”). The resulting MEA was assembled into a fuel cell set FC25-02SPmanufactured by ElectroChem. Inc. Power generation test was performed ata cell temperature of 70° C., a hydrogen utilization of 50%, an oxygenutilization of 25%, a hydrogen humidifying temperature of 70° C., and anoxygen humidifying temperature of 30° C. A power generation curve thusobtained is shown in FIG. 2. Further, with the use of an electrode cellmanufactured by ElectroChem. Inc. as a gas diffusion electrode, whichwas loaded with 20 wt % of Pt and 10 wt % of Ru, another powergeneration test was performed using 10 wt % methanol solution in placeof hydrogen at room temperature and an oxygen utilization of 25% withouthumidification for oxygen. A cell voltage of 0.25 V was obtained at acurrent density of 0.1 A/cm².

Example 7 Polymerization of Block Polymer PB7

In a flask were charged 28.7 g of bis(4-chlorophenyl)sulfone, 18.9 g of4,4′-biphenol, and 16.8 g of potassium carbonate. After 160 ml ofdimethylsulfoxide and 50 ml of toluene were added, the reaction mixturewas heated and stirred under nitrogen flow. The temperature was elevatedto 175° C. while removing the generating water together with toluene,and then the reaction mixture was stirred at that temperature for 16hours to prepare a polymer solution A. Separately, a solution wasprepared by dissolving 73 g of polyethersulfone (“SUMIKAEXCEL 7600P”,manufactured by Sumitomo Chemical Co., Ltd.) in 290 mL ofdimethylsulfoxide. The solution was poured into the polymer solution A,and then the mixed solution was stirred at 170° C. for 1.5 hours. Thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in hot water twice, and in methanol once, whereby a blockcopolymer PB7 was obtained. The solution viscosity η_(sp/c) of theobtained polymer was 0.71 dl/g.

Sulfonation of Block Polymer PB7 (SPB7 Synthesis)

In 200 ml of 98% sulfuric acid, 20 g of the block polymer PB7 weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a sulfonated blockcopolymer SPB7 that was composed of a hydrophilic segment substantiallyconsisting of the structural unit represented by the following formula(20) and a hydrophobic segment consisting of the structural unitrepresented by the following formula (21). The ion-exchange capacity ofthe obtained polymer was 1.63 mmol/g. If SPB7 is a blend, theion-exchange capacity is largely lowered by water washing aftersulfonation, because the water-soluble hydrophilic polymer is removedand the ion-exchange capacity is largely lowered. However, theion-exchange capacity of SPB7 gave almost the same value as thecalculated value, 1.71 mmol/g obtained by assuming that one sulfonicacid group was incorporated into each of the biphenol residue aromaticrings of PB7 before sulfonation. This indicates that SPB7 is not a blendand that the hydrophilic segment and hydrophobic segment are bondedtogether. In addition, in the membrane of SPB7 alone that was preparedby the method described below, a phase-separation structure was shown byTEM observation. Hence SPB7 is considered to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions. Only the hydrophilic segment is considered to have astructure with sulfonic acid groups. Based on the SPB7 compositionobtained by the H-NMR measurement, the hydrophilic segment weightfraction after the sulfonic acid groups were converted to SO₃H was 0.48.The ion-exchange capacity of the hydrophilic segment of SPB7 was 3.4mmol/g.

<Membrane Preparation>

A solution with a solid content of 20 wt % was prepared by dissolving inN,N-dimethylacetoamide, so that the composition ratio (weight) of thesulfonated block copolymer SPB7 thus obtained to a polyethersulfone(“SUMIKAEXCEL 4100P”, manufactured by Sumitomo Chemical Co., Ltd.)having the same structural unit as that of the hydrophobic segment ofSPB7 results in 9:1. The solution was cast on a glass plate, and thendried with hot air at 120° C. for 1 hour. The resulting membrane waspeeled off from the glass plate, fixed on a stainless steel frame, andfurther heated and dried at 200° C. for 30 minutes. Thus obtainedmembrane was immersed in a 0.5 N NaOH aqueous solution for 2 hours atroom temperature, further in a 1 N H₂SO₄ aqueous solution for 2 hours,water-washed, and then fixed and dried on a stainless steel frame toobtain a 30 μm thick polymer electrolyte membrane. Similarly, thepreparation and treatment of a membrane were carried out using only SPB1to obtain a 32 μm thick SPB7 membrane. The properties of the membraneprepared from the composition are shown in Table 2.

Example 8

Polymerization of Polyethersulfone Block Copolymer PB8

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 12.71 g ofbis(4-fluorophenyl)sulfone, 22.91 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17 g of potassium carbonate were charged; after 200ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 175° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a hydrophilic prepolymer HP8solution. Separately, 100 g of “SUMIKAEXCEL 7600P” having the structuralunit represented by the following formula (22) were dissolved in 310 mlof dimethylsulfoxide; and then 50 ml of toluene were added so as toobtain a hydrophobic prepolymer SP8 solution that was azeotropicallydehydrated. The SP8 solution was added to the HP8 solution, and then themixed solution was stirred at 160° C. for 2 hours. The solution waspoured into a large amount of water to deposit a white solid product,which was then filtered off. Thus obtained solid product was washed inhot water twice, and in methanol once, whereby a block copolymer PB8 wasobtained. The solution viscosity η_(sp/c) of the obtained polymer was0.65 dl/g. The ion-change capacity was 0.79 mmol/g.

Sulfonation of Block Copolymer PB8 (SPB8 Synthesis)

In 200 ml of 98% sulfuric acid, 20 g of block polymer PB8 weredissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times to obtain asulfonated block copolymer SPB8 composed of a hydrophilic segment havingthe random copolymer structure represented by the following formula (23)and a hydrophobic segment having the structural unit represented by thefollowing formula (24). The ion-exchange capacity of the obtainedpolymer was 1.80 mmol/g. If the product is a blend, the water-solublehydrophilic polymer is removed by water washing after sulfonation, andthe ion-exchange capacity is largely lowered. However, the ion-exchangecapacity of SPB8 gave almost the same value as the calculated value,1.84 mmol/g obtained by assuming that one sulfonic acid group wasincorporated into each of the biphenol residue aromatic rings of PB8before sulfonation. This indicates that SPB8 is not a blend, and thatthe hydrophilic segment and hydrophobic segment are bonded together. Inaddition, in the membrane of SPB8 alone that was prepared in the samemanner as described below, a phase-separation structure was shown by TEMobservation. Hence, SPB8 was identified to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions, so that SPB8 has such a structure that only the hydrophilicsegment has sulfonic acid groups. Based on the SPB8 composition obtainedby the H-NMR measurement, the hydrophilic segment weight fraction afterthe sulfonic acid groups were converted to SO₃H was 0.39. Theion-exchange capacity of the hydrophilic segment of SPB8 was 4.6 mmol/g.

wherein, p and q represent the mol fraction of each structural unitrespectively; and p=0.5 and q=0.5.

<Membrane Preparation>

A composition solution was prepared by dissolving inN,N-dimethylacetoamide (solid content: 20 wt %), so that the compositionratio (weight) of SPB8 to a polyethersulfone (“SUMIKAEXCEL 7600P”,manufactured by Sumitomo Chemical Co., Ltd.) having the same structureas that of the hydrophobic segment of SPB8 results in 8:2. The solutionwas then cast on a glass plate and dried at 120° C. for 1 hour. Theresulting self-supporting membrane was peeled off from the glass plate,fixed on a stainless steel frame, further dried with hot air at 200° C.for 0.5 hour to obtain a 28 μm thick membrane. The resulting membranewas immersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, immersed in a 1 N sulfuric acidaqueous solution for 4 hours, and then water-washed 3 times. After thewashing water was confirmed to be neutral, the membrane was fixed on astainless steel frame and dried. The properties of the membrane preparedfrom the composition are shown in Table 2.

Comparative Example 3

A 32 μm thick polymer electrolyte membrane was prepared in the samemanner as in Example 2, except that a polysulfone (available fromAldrich, 26,000 of number average molecular weight) that has thestructure represented by the following formula (25) was used in place ofthe polyethersulfone (“SUMIKAEXCEL 7600P”, manufactured by SumitomoChemical Co., Ltd.). The properties of the resulting membrane are shownin the table. As compared with Example 2, the proton conductivity waslargely lowered at low humidity, and the properties on water absorptionwere less improved.

Example 9 Polymerization of Polyethersulfone Block Copolymer PB9

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 6.36 g ofbis(4-fluorophenyl)sulfone, 34.37 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17 g of potassium carbonate were charged; after 200ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 175° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a hydrophilic prepolymer HP9solution. Separately, 86.7 g of “SUMIKAEXCEL 7600P” were dissolved in340 ml of dimethylsulfoxide. To the resulting solution, 50 ml of toluenewere added so as to obtain a hydrophobic prepolymer SP9 solution thatwas azeotropic ally dehydrated. The SP9 solution was added to the HP9solution, and then the mixed solution was stirred at 160° C. for 2.5hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB3 was obtained. The solution viscosity η_(sp/c) of theobtained polymer was 0.70 dl/g. The ion-change capacity was 0.99 mmol/g.

Sulfonation of Block Copolymer PB9 (SPB9 Synthesis)

In 200 ml of 98% sulfuric acid, 20 g of block copolymer PB9 weredissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a sulfonated blockcopolymer SPB9 having a hydrophilic segment with the random copolymerstructure represented by the following formula (26) and a hydrophobicsegment with the structural unit represented by the following formula(27). The ion-exchange capacity of the polymer thus obtained was 2.01mmol/g.

wherein, p and q represent the mole fraction of each structural unitrespectively; and p=0.75 and q=0.25.

The hydrophilic segment, which is water-soluble when it acts solely, isremoved by water washing, and the ion-exchange capacity is loweredlargely. However, the ion-exchange capacity of SPB9 gave almost the samevalue as the calculated value, 2.27 mmol/g obtained by assuming that onesulfonic acid group was incorporated into each of the biphenol residuearomatic rings of PB9 before sulfonation. This indicates that thehydrophilic segment and hydrophobic segment are bonded together.

If the product is a blend, the water-soluble hydrophilic polymer isremoved and the ion-exchange capacity is largely lowered by waterwashing after sulfonation. However, the ion-exchange capacity of SPB9gave almost the same value as the calculated value, 2.27 mmol/g obtainedby assuming that one sulfonic acid group was incorporated into each ofthe biphenol residue aromatic rings of PB9 before sulfonation. Thisindicates that SPB9 is not a blend, and that the hydrophilic segment andhydrophobic segment are bonded together. In addition, in the membrane ofSPB9 alone prepared in the same manner as described below, aphase-separation structure was observed by TEM observation. Hence, SPB9was identified to be a block copolymer. The hydrophobic segment is notsulfonated under the foregoing sulfonation conditions, so that SPB9 hassuch a structure that only the hydrophilic segment has sulfonic acidgroups. Based on the SPB9 composition obtained by the H-NMR measurement,the hydrophilic segment weight fraction after the sulfonic acid groupswere converted to SO₃H was 0.43. The ion-exchange capacity of thehydrophilic segment of SPB9 was 4.54 mmol/g.

<Membrane Preparation>

A composition solution was prepared by dissolving inN,N-dimethylacetoamide (solid content: 20 wt %), so that the compositionratio (weight) of SPB9 to a polyethersulfone (“SUMIKAEXCEL 4100P”,manufactured by Sumitomo Chemical Co., Ltd.) having the same structureas that of the hydrophobic segment results in 8:2. The solution was thencast on a glass plate and dried at 120° C. for 1 hour. The resultingself-supporting membrane was peeled off from the glass plate, fixed on astainless steel frame, further dried with hot air at 200° C. for 0.5hour to obtain a 28 μm thick membrane. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, immersed in a 1 N sulfuric acidaqueous solution for 4 hours, and then water-washed 3 times. After thewashing water was confirmed to be neutral, the membrane was fixed on astainless steel frame and dried. The properties of the membrane preparedfrom the composition are shown in Table 2.

Comparative Example 4

A sulfonated block copolymer SPB9′ having the same hydrophilic segmentand hydrophobic segment as the ones in Example 9 and a different weightratio of the hydrophobic segment to hydrophilic segment was synthesizedin the same manner as in Example 9, except that 158 g of “SUMIKAEXCEL7600P” were dissolved in 640 ml of dimethylsulfoxide in the synthesis ofthe block copolymer PB9 in Example 9. The ion-exchange capacity of theobtained SPB9′ was 1.54 mmol/g and was identified to be a blockcopolymer in the same manner as in Example 9. A membrane composed ofSPB9′ solely was prepared in the same manner as in Example 9. Theproperties of the membrane are shown in the table. As compared with themembrane in Example 9 having the same ion-exchange capacity, the protonconductivity at low humidity was lower.

Example 10

SPB9 synthesized in Example 9, and the block copolymer PB7 beforesulfonation that was synthesized in Example 7 and had a segment composedof the same structural unit as that of the hydrophobic segment of SPB9were dissolved in a SPB9:PB7 weight ratio of 9:1 inN,N-dimethylacetoamide (solid content: 20 wt %). The resulting solutionwas cast on a glass plate and dried at 120° C. for 1 hour. The resultingself-supporting membrane was peeled off from the glass plate, fixed on astainless steel frame, further dried with hot air at 200° C. for 0.5hour to obtain a 28 μm thick membrane. The resulting membrane wasimmersed in a 1 N sodium hydroxide aqueous solution at room temperaturefor 2 hours, water-washed, and then immersed in a 1 N sulfuric acidaqueous solution for 4 hours. The membrane was water-washed 3 times.After the washing water was confirmed to be neutral, the membrane wasfixed on a stainless steel frame and dried. The properties of theobtained membrane are shown in Table 2.

Example 11 Polymerization of Polyethersulfone Block Copolymer PB11

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 45.83 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17 g of potassium carbonate were charged; after 200ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 175° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a hydrophilic prepolymer HP11solution. Separately, 99.2 g of “SUMIKAEXCEL 7600P” were dissolved in395 ml of dimethylsulfoxide; and then 50 ml of toluene were added so asto prepare a hydrophobic prepolymer SP11 solution that wasazeotropically dehydrated. The SP11 solution was added to the HP11solution, and then the mixed solution was stirred at 160° C. for 2.5hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB11 was obtained. The solution viscosity η_(sp/c) ofthe obtained polymer was 0.63 dl/g. The ion-change capacity was 1.18mmol/g.

Sulfonation of Block Copolymer PB11 (SPB11 Synthesis)

In 200 ml of 98% sulfuric acid, 20 g of block copolymer PB11 weredissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times to obtain asulfonated block copolymer SPB11 composed of a hydrophilic segmenthaving the structural unit represented by the following formula (28) anda hydrophobic segment having the structural unit represented by thefollowing formula (29). The ion-exchange capacity of the obtainedpolymer was 2.2 mmol/g.

The hydrophilic segment, which is water-soluble when it acts solely, isremoved by water washing and the ion-exchange capacity is loweredlargely. However, the ion-exchange capacity of SPB11 gave almost thesame value as the calculated value, 2.35 mmol/g obtained by assumingthat one sulfonic acid group was incorporated into each of the biphenolresidue aromatic rings of PB11 before sulfonation. This indicates thatthe hydrophilic segment and hydrophobic segment are bonded together.

If the product is a blend, the water-soluble hydrophilic polymer isremoved and the ion-exchange capacity is largely decreased by waterwashing after sulfonation. However, the ion-exchange capacity of SPB11gave almost the same value as the calculated value, 2.35 mmol/g obtainedby assuming that one sulfonic acid group was incorporated into each ofthe biphenol residue aromatic rings of PB11 before sulfonation. Thisindicates that SPB11 is not a blend, and that the hydrophilic segmentand hydrophobic segment are bonded together. In addition, in themembrane of SPB11 alone that was prepared in the same manner asdescribed below, a phase-separation structure was observed by TEMobservation. Hence, SPB11 was identified to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions, so that SPB11 has such a structure that only the hydrophilicsegment has sulfonic acid groups. Based on the SPB11 compositionobtained by the H-NMR measurement, the hydrophilic segment weightfraction after the sulfonic acid groups were converted to SO₃H was 0.42.The ion-exchange capacity of the hydrophilic segment of SPB11 was 5.2mmol/g.

<Membrane Preparation>

A composition solution was prepared by dissolving inN,N-dimethylacetoamide (solid content: 20 wt %), so that the compositionratio (weight) of SPB11 to a polyethersulfone (“SUMIKAEXCEL 4100P”,manufactured by Sumitomo Chemical Co., Ltd.) having the same structureas that of the hydrophobic segment results in 9:1. The solution was thencast on a glass plate and dried at 120° C. for 1 hour. The resultingself-supporting membrane was peeled off from the glass plate, fixed on astainless steel frame, further dried with hot air at 200° C. for 0.5hour to obtain a 28 μm thick membrane. The membrane was immersed in a0.5 N sodium hydroxide aqueous solution at room temperature for 2 hours,water-washed, immersed in a 1 N sulfuric acid aqueous solution for 4hours, and then water-washed 3 times. After the washing water wasconfirmed to be neutral, the membrane was fixed on a stainless steelframe and dried. The properties of the membrane prepared from thecomposition are shown in Table 2.

Comparative Example 5 Synthesis of Random Copolymer PR1

25.42 g of bis(4-fluorophenyl)sulfone, 15.93 g of bis(4-hydroxyphenyl)sulfone, and 6.77 g of 4,4′-biphenol were charged; after 170 ml ofdimethylsulfoxide and 50 ml of toluene were added, the reaction mixturewas heated and stirred under a nitrogen gas atmosphere. The temperaturewas elevated to 175° C. while removing the generating water togetherwith toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a polymer solution. The solution waspoured into a large amount of water to deposit a white solid product,which was then filtered off. Thus obtained solid product was washed inhot water twice, and in methanol once, whereby a polymer PR1 wasobtained. The solution viscosity η_(sp/c) of the obtained polymer PR1was 0.53 dl/g.

Sulfonation of Random Copolymer PR1

In 200 ml of 98% sulfuric acid, 21 g of random copolymer PR1 weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times to obtain a polymerSPR1. The ion-exchange capacity of the polymer was 1.47 mmol/g. In theSPR1 membrane prepared in the same manner as described below, aphase-separation structure was not observed by TEM observation, and themembrane was observed to be uniform. Hence, the polymer was identifiedto be a random copolymer.

<Membrane Preparation>

A composition solution was prepared by dissolving inN,N-dimethylacetoamide (solid content: 20 wt %), so that the compositionratio (weight) of SPR1 and a polyethersulfone (“SUMIKAEXCEL 4100P”,manufactured by Sumitomo Chemical Co., Ltd.) having the same structureresults in 9:1. The solution was then cast on a glass plate and dried at120° C. for 1 hour. The resulting self-supporting membrane was peeledoff from the glass plate, fixed on a stainless steel frame, furtherdried with hot air at 200° C. for 0.5 hour to obtain a 28 μm thickmembrane. The resulting membrane was immersed in a 0.5 N sodiumhydroxide aqueous solution at room temperature for 2 hours,water-washed, immersed in a 1 N sulfuric acid aqueous solution for 4hours, and then water-washed 3 times. After the washing water wasconfirmed to be neutral, the membrane was fixed on a stainless steelframe and dried. The properties of the membrane prepared from thecomposition are shown in Table 2.

TABLE 2 Mechanical properties on Proton conductivity water-absorptionIon-exchange (70° C.) Elas- Strength capacity 30% RH 90% RH ticity atbreak (mmol/g) (mS/cm) (mS/cm) (GPa) (MPa) Example 7 1.31 1.1 140 0.2327 Percent of 73 100 190 180 retention (%) Example 8 1.44 2.2 150 0.2725 Percent of 55 75 180 132 retention (%) Compara- 1.44 1.1 150 0.20 17tive Percent of 28 75 133 89 Example 3 retention (%) Ratio to 0.5 1.00.74 0.68 Example 2 Example 9 1.51 4.3 200 0.25 26 Percent of 73 57 139130 retention (%) Compara- 1.54 1.53 180 0.22 22 tive Ratio to 0.36 0.90.88 0.85 Example 4 Example 3 Example 10 1.70 5.1 220 0.22 23 Percent of87 63 122 115 retention (%) Example 11 1.98 5.9 230 0.21 23 Percent of80 100 131 147 retention (%) Compara- 1.32 0.5 130 0.21 21 tive Ratio to0.45 0.93 0.91 0.78 Example 5 Example 1 (1) The percent of retention wasobtained from 100 × D2/D1, wherein D1 is each property value of amembrane prepared from a sulfonated polymer alone, and D2 is eachproperty value of a composition. (2) In each Comparative Example, theratio to the corresponding Example was obtained from D4/D3, wherein D3is each property value of the corresponding Example, and D4 is eachproperty value of corresponding Comparative Example.

Synthesis Example 2 Synthesis of3,3′-Disulfo-4,4′-Dichlorodiphenylsulfone Potassium Salt

In a flask 120 g of bis(4-chlorophenyl)sulfone and 250 g of 30% fumingsulfuric acid were charged, and heated at 110° C. for 6 hours whilestirring. After the resulting solution was gradually added to ice water,potassium chloride was added so as to deposit a solid product. The solidproduct was dissolved again in water, and then potassium chloride wasadded so as to deposit a solid product. The solid product was filteredoff, recrystallized twice with an ethanol/water (6/4) mixture, and driedto obtain a white solid product. Thus obtained white solid productprovided H-NMR signals at 7.66 ppm, 7.84, and 8.35 ppm with theintegrated intensities of 1:1:1. The white solid product was identifiedto be 3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt.

Synthesis Example 3 Synthesis of3,3′-Disulfo-4,4′-Dichlorodiphenylsulfone Sodium Salt

In a flask 120 g of bis(4-chlorophenyl)sulfone and 250 g of 30% fumingsulfuric acid were charged, and heated at 110° C. for 6 hours whilestirring. After the resulting solution was gradually added to ice water,sodium chloride was added so as to deposit a solid product. The solidproduct was dissolved again in water. After the resulting solution wasneutralized with NaOH, sodium chloride was added so as to deposit asolid product again. The solid product was recrystallized twice with anethanol/water (4/1) mixture and dried to obtain a white solid product.Thus obtained white solid product provided H-NMR signals at from 7.6 to7.7 ppm, from 7.8 to 7.9 ppm, and from 8.3 to 8.4 ppm with theintegrated intensities of 1:1:1. The white solid product was identifiedto be 3,3′-disulfo-4,4′-dichlorodiphenylsulfone sodium salt.

Example 12 Synthesis of Polyethersulfone Block Copolymer PB12

In a four-neck separable flask equipped with a stirrer, a Dean-Starktrap, a thermometer, and a nitrogen gas inlet, 3.35 g (0.012 mol) ofbis(4-chlorophenyl) sulfone, 18.34 g (0.035 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, 8.79 g (0.047mol) of 4,4′-biphenol, and 8.22 g of potassium carbonate were charged;after 80 g of dimethylsulfoxide and 35 g of toluene were added, thereaction mixture was heated and stirred under nitrogen flow. Thetemperature was elevated to 160° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 8 hours to prepare a hydrophilic segment prepolymer HP12solution. Here, the ratio of 4,4′-biphenol that is a dihydric phenol tothe aromatic dichloride was 1.01:1. The resulting prepolymerconcentration with respect to the total amount of the prepolymer anddimethylsulfoxide in the HP12 solution was 25.2 wt %. Separately, 40.38g of “SUMIKAEXCEL 7600P” were dissolved in 160 g of dimethylsulfoxideand 80 g of toluene, and then the resulting solution was heated andstirred under nitrogen flow. The temperature was elevated to 185° C.while removing the flowing water together with toluene, and then thesolution was stirred at that temperature for 8 hours to prepare ahydrophobic segment prepolymer SP12 solution. The concentration of theprepolymer SP12 with respect to the total amount of the prepolymer SP12and dimethylsulfoxide in the SP12 solution was 20.2 wt %. The SP12solution was added to the HP12 solution, and then the mixed solution wasstirred at 160° C. for 2 hours. The solution was poured into a largeamount of water to deposit a white solid product, which was thenfiltered off. Thus obtained solid product was washed in hot water twice,and in methanol once, whereby a block copolymer PB12 was obtained. Thesolution viscosity η_(sp/c) of the obtained polymer was 1.05 dl/g. Theion-exchange capacity was 0.92 mmol/g.

Sulfonation of Block Copolymer PB12 (SPB12 Synthesis)

In 332 g of 98% sulfuric acid 25 g of the block copolymer PB12 weredissolved, and the resulting solution was stirred at 40° C. for 48hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidwas washed in water 5 times to obtain a polymer SPB12. The ion-exchangecapacity of the polymer was 2.00 mmol/g. If the polymer is a blend, thehydrophilic polymer that is water-soluble is removed by water washingafter sulfonation and the ion-exchange capacity is largely lowered.However, the ion-exchange capacity of SPB12 gave almost the same valueas the calculated value, 2.08 mmol/g obtained by assuming that onesulfonic acid group was incorporated into each of the biphenol residuearomatic rings of PB12 before sulfonation. This indicates that SPB12 isnot a blend, but the hydrophilic segment and hydrophobic segment arebonded together. In addition, in the membrane prepared by the methoddescribed below, a phase-separation structure was observed by TEMobservation. Hence, SPB12 was identified to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions. Therefore, only the hydrophilic segment is considered tohave sulfonic acid groups. Based on the SPB12 composition obtained bythe H-NMR measurement, the hydrophilic segment weight fraction after thesulfonic acid groups were converted to SO₃H was 0.41. The ion-exchangecapacity of the hydrophilic segment of SPB12 was 4.88 mmol/g.

SPB12 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 120° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, and then immersed in a 1 Nsulfuric acid aqueous solution for 4 hours. The membrane waswater-washed 3 times. After the washing water was confirmed to beneutral, the membrane was fixed on a metal frame and dried at 40° C. toobtain a 32 μm thick membrane. The proton conductivity was measured at70° C. with changing the relative humidity. The results are shown inTable 3.

Example 13 Synthesis of Prepolymer HP13

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 8.41 g (0.045 mol) of4,4′-biphenol and 50 g of N,N-dimethylacetoamide were charged. They werestirred at 60° C. with nitrogen gas bubbling and dissolved. After 7.87 gof potassium carbonate and 15 g of toluene were added, they were heatedand stirred at 160° C. under nitrogen flow so as to remove thegenerating water together with toluene. After that, 6.38 g (0.022 mol)of bis(4-chlorophenyl)sulfone, 11.64 g (0.022 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, and 60 g ofN,N-dimethylacetoamide were added. The resulting reaction mixture wasstirred at 160° C. for 16 hours under nitrogen flow to obtain ahydrophilic segment prepolymer HP13 solution. Here, the ratio of4,4′-biphenol that is a dihydric phenol to the aromatic dichloride was1.016:1. The resulting prepolymer concentration with respect to thetotal amount of the prepolymer and N,N-dimethylacetoamide was 18 wt %.After insoluble substance was filtered off, the resulting filtrate waspoured into a large amount of 2-propanol to deposit a white solidproduct, which was then vacuum-dried at 100° C. and dissolved again inN,N-dimethylacetoamide. The processes of filtration, deposition with2-propanol, and drying were repeated to obtain a prepolymer HP13 havingsulfonic acid groups in potassium form. The η_(sp/c) of HP13 was 0.89dl/g. The ion-exchange capacity was 1.91 mmol/g.

Synthesis of Block Copolymer PB13

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 6.05 g of HP13 and 55 g ofN,N-dimethylacetoamide were charged. They were stirred at 80° C.overnight under nitrogen flow so as to dissolve HP13. After 0.029 g ofpotassium carbonate and 10 g of toluene added, the solution was heatedand stirred at 160° C. under nitrogen flow to remove the generatingwater together with toluene. Separately, 7.36 g of “SUMIKAEXCEL 7600P”were dissolved in 36 g of N,N-dimethylacetoamide by stirring them at 80°C. overnight under nitrogen flow. Similarly, 10 g of toluene were addedso as to dehydrate the resulting solution. In this way, a hydrophobicsegment prepolymer SP13 solution was obtained. The SP13 solution wasadded to the HP13 solution, and then the mixed solution was stirred at160° C. for 2 hours. After insoluble substance was filtered off, theresulting filtrate was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB13 was obtained. The solution viscosity η_(sp/c) ofthe obtained polymer was 1.09 dl/g. The ion-exchange capacity was 0.643mmol/g.

Sulfonation of Block Copolymer PB13 (SPB13 Synthesis)

In 63.6 g of 95% sulfuric acid 7.07 g of the block copolymer PB13 wasdissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a polymer SPB13. Ifthe polymer is a blend, the hydrophilic polymer that is water-soluble isremoved by water washing after sulfonation and the yield is largelylowered. However, the yield of SPB13 was 94% of the calculated valueobtained by assuming that one sulfonic acid group was incorporated intoeach of the biphenol residue aromatic rings of PB13 before sulfonation.This indicates that SPB13 is not a blend, but the hydrophilic segmentand hydrophobic segment are bonded together. The ion-exchange capacitywas 1.72 mmol/g. In addition, in the membrane prepared by the methoddescribed below, a phase-separation structure was observed by TEMobservation. Hence, SPB13 was identified to be a block copolymer. Thehydrophobic segment is not sulfonated under the foregoing sulfonationconditions. Therefore, only the hydrophilic segment is considered tohave sulfonic acid groups. Based on the SPB13 composition obtained bythe H-NMR measurement, the hydrophilic segment weight fraction after thesulfonic acid groups were converted to SO₃H was 0.46. The ion-exchangecapacity of the hydrophilic segment of SPB13 was 4.20 mmol/g.

SPB13 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution, which was then cast on a glass plate and dried at 120° C. for1 hour. After the resulting self-supporting membrane was peeled off fromthe glass plate and fixed on a metal frame, the membrane was furtherdried with hot air at 200° C. for 0.5 hour. The resulting membrane wasimmersed in a 0.5 N sodium hydroxide aqueous solution at roomtemperature for 2 hours, water-washed, and then immersed in a 1 Nsulfuric acid aqueous solution for 4 hours. The membrane waswater-washed 3 times. After the washing water was confirmed to beneutral, the membrane was fixed on a metal frame and dried at 40° C. toobtain a 21 μm thick membrane. The proton conductivity was measured at70° C. with changing the relative humidity. The results are shown inTable 3.

Example 14 Synthesis of Prepolymer HP14

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 8.93 g (0.048 mol) of4,4′-biphenol and 80 g of 1,3-dimethyl-2-imidazolidinone were charged.They were stirred at 60° C. with nitrogen gas bubbling and dissolved.After 20 ml (0.1 mol NaOH) of a 4.8 N sodium hydroxide aqueous solutionwas added, and the mixture was stirred at 100° C. for 1 hour to obtain auniform solution. After 40 ml of toluene was added, the solution washeated and stirred at 140° C. to 170° C. under nitrogen flow so as toremove the generating water together with toluene. After that, 9.38 g(0.033 mol) of bis(4-chlorophenyl)sulfone, 7.12 g (0.0145 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone sodium salt, and 40 g of1,3-dimethyl-2-imidazolidinone were added. The resulting reactionmixture was stirred at 180° C. for 16 hours under nitrogen flow toobtain a hydrophilic segment prepolymer HP14 solution. Here, the ratioof 4,4′-biphenol that is a dihydric phenol to the aromatic dichloridewas 1.016:1. The resulting prepolymer concentration with respect to thetotal amount of the prepolymer and 1,3-dimethyl-2-imidazolidinone in theHP14 solution was 15 wt %. After insoluble substance was filtered off,the resulting filtrate was poured into a large amount of 2-propanol todeposit a solid product, which was then vacuum-dried at 100° C. Thesolid product was washed with 2-propanol and ethanol repeatedly, andthen vacuum-dried at 100° C. After the resulting solid was stirred in 1N sulfuric acid, the solid was washed with a large amount of water andvacuum-dried at 100° C. to obtain a prepolymer HP14 having sulfonic acidgroups in H form. The η_(sp/c) of HP14 was 0.31 dl/g. The ion-exchangecapacity was 1.30 mmol/g.

Synthesis of Block Copolymer PB14

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 5.0 g of a prepolymer HP14 havingsulfonic acid group in H form and 25 g of dimethylsulfoxide werecharged. They were stirred at 120° C. under nitrogen flow so as todissolve the HP14. After 1.04 g of potassium carbonate and 5 g oftoluene added, the solution was heated and stirred at 175° C. undernitrogen flow to remove the generating water together with toluene.Separately, 7.5 g of “SUMIKAEXCEL 7600P” were dissolved in 30 g ofdimethylsulfoxide by stirring them at 170° C. under nitrogen flow.Similarly, the resulting solution was dehydrated by adding 5 g oftoluene so as to obtain a hydrophobic segment prepolymer SP14 solution.To the SP14 solution were added the HP14 solution and 4 g ofdimethylsulfoxide, and then the mixed solution was stirred at 175° C.for 2 hours. After insoluble substance was removed by filtration, theresulting filtrate was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidproduct was washed in hot water twice, and in methanol once, whereby ablock copolymer PB14 was obtained. The ion-exchange capacity of theobtained polymer was 0.21 mmol/g.

Sulfonation of Block Copolymer PB14 (SPB14 Synthesis)

In 36 g of 95% sulfuric acid 4 g of the block copolymer PB14 wasdissolved, and the resulting solution was stirred at room temperaturefor 48 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a polymer SPB14.The ion-exchange capacity of the obtained polymer was 1.68 mmol/g. Ifthe polymer is a blend, the hydrophilic polymer that is water-soluble isremoved by water washing after sulfonation and the ion-exchange capacityis largely lowered. However, the ion-exchange capacity of SPB14 gavealmost the same value as the calculated value, 1.77 mmol/g obtained byassuming that one sulfonic acid group was incorporated into each of thebiphenol residue aromatic rings of PB14 before sulfonation. Thisindicates that SPB14 is not a blend, and that the hydrophilic segmentand hydrophobic segment are bonded together. In addition, in themembrane prepared by the method described below, a phase-separationstructure was observed by TEM observation. Hence, SPB14 was identifiedto be a block copolymer. The hydrophobic segment is not sulfonated underthe foregoing sulfonation conditions. Therefore, only the hydrophilicsegment is considered to have sulfonic acid groups. Based on the SPB14composition obtained by the H-NMR measurement, the hydrophilic segmentweight fraction after the sulfonic acid groups were converted to SO₃Hwas 0.43. The ion-exchange capacity of the hydrophilic segment of SPB14was 3.91 mmol/g.

Comparative Example 6

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 13.58 g (0.028 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone sodium salt in place of3,3′-disulfo-4,4′-dichlorodiphenylsulfone potassium salt, 7.94 g (0.028mol) of bis(4-chlorophenyl)sulfone, 10.46 g (0.0562 mol) of4,4′-biphenol, and 9.78 g of potassium carbonate were charged. After 100g of dimethylsulfoxide and 45 g of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 160° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 8 hours to prepare a hydrophilic segment prepolymerHP14′ solution. Here, the ratio of 4,4′-biphenol that is a dihydricphenol to the aromatic dichloride was 1.016:1. The resulting prepolymerconcentration with respect to the total amount of the prepolymer anddimethylsulfoxide in the HP14′ solution was 21.8 wt %.

Separately, 38.52 g of “SUMIKAEXCEL 7600P” were dissolved in 160 g ofdimethylsulfoxide and 70 g of toluene, and then the resulting solutionwas heated and stirred under nitrogen flow. The temperature was elevatedto 185° C. while removing the flowing water together with toluene, andthen the solution was stirred at that temperature for 8 hours to preparea hydrophobic segment prepolymer SP14′ solution. The concentration ofthe prepolymer SP14′ with respect to the total amount of the prepolymerSP14′ and dimethylsulfoxide was 20.0 wt %. The SP14′ solution was addedto the HP14′ solution, and then the mixed solution was stirred at 160°C. for 2 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in hot water twice, and in methanolonce, whereby a polymer PB14′ was obtained. The solution viscosity ηsp/c of the obtained polymer was 0.52 dl/g.

In 90 g of 98% sulfuric acid, 10 g of the polymer PB14′ were dissolved,and the resulting solution was stirred at room temperature for 48 hours.The solution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid productwas washed in water 5 times to obtain a polymer SPB14′. The ion-exchangecapacity of SPB14′ was not able to be measured by titration. This isbecause, in the case where HP14′ having sulfonic acid group in sodiumsalt form were used, block copolymerization did not proceed, and almostall of the water-soluble hydrophilic polymer having sulfonic acid groupsincorporated therein were removed by water washing after sulfonation. Inaddition, in the H-NMR measurement, the signals derived from thehydrophilic segment were hardly observed.

Comparative Example 7

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 8.924 g (0.048 mol) of4,4′-biphenol and 50 g of dimethylsulfoxide were charged. They werestirred at 60° C. with nitrogen gas bubbling and dissolved. After 20 ml(0.096 mol of NaOH) of a 4.794 N sodium hydroxide aqueous solution wasadded, and the mixture was stirred at 100° C. for 1 hour to obtain auniform solution. After 60 ml of toluene was added, the solution washeated and stirred at 150° C. to 170° C. under nitrogen flow so as toremove the generating water together with toluene. After that, asolution prepared by dissolving 6.77 g (0.0236 mol) ofbis(4-chlorophenyl)sulfone and 11.585 g (0.0236 mol) of3,3′-disulfo-4,4′-dichlorodiphenylsulfone sodium salt in 40 g ofdimethylsulfoxide was added. Separately, another 10 g ofdimethylsulfoxide were also added. The resulting reaction mixture wasstirred at 180° C. for 1616 hours under nitrogen flow to obtain ahydrophilic segment prepolymer HP15′ solution. Here, the ratio of4,4′-biphenol that is a dihydric phenol to the aromatic dichloride was1.016:1. The resulting prepolymer concentration in the HP15′ solutionwith respect to the total amount of the prepolymer and dimethylsulfoxidewas 19 wt %.

Separately, 33.5 g of “SUMIKAEXCEL 7600P” were dissolved in 134 g ofdimethylsulfoxide. After 50 ml of toluene was added, the resultingmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature of the mixture was elevated to 170° C. while removing theflowing water together with toluene, and further stirred at thattemperature for 2 hours to obtain a hydrophobic segment prepolymer SP15′solution. The prepolymer SP15′ concentration in the SP15′ solution withrespect to the total amount of the prepolymer and dimethylsulfoxide was20 wt %. To the SP15′ solution, the HP15′ solution and 20 g ofdimethylsulfoxide were added, and then the resulting mixture was stirredat 160° C. to 170° C. for 3 hour. After insoluble substance was removedby filtration, the resulting filtrate was poured into a large amount ofwater to deposit a white solid product, which was then filtered off.Thus obtained solid product was washed in hot water twice, and inmethanol once, whereby a polymer PB15′ was obtained.

In 18 g of 95% sulfuric acid, 2 g of the polymer PB15′ were dissolved,and the resulting solution was stirred at room temperature for 24 hours.The solution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. Thus obtained solid waswashed in water 5 times to obtain a polymer SPB15′. The ion-exchangecapacity of the obtained polymer was 0.209 mmol/g. The ion-exchangecapacity was largely lowered as compared with the calculatedion-exchange capacity of 1.89 mmol/g obtained by assuming that onesulfonic acid group was incorporated into each of the biphenol residuearomatic rings of PB15′ before sulfonation. In this case, blockcopolymerization did not proceed and PB15′ was formed as a blend,because HP15′ having sulfonic acid groups in sodium salt form were usedfor the block copolymerization. This indicates that much of thewater-soluble hydrophilic polymer was removed by water washing aftersulfonation.

Referenced Example 3

In 180 g of 98% sulfuric acid, 20 g of “SUMIKAEXCEL 7600P” weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times. The ion-exchangecapacity of the obtained polymer was not able to be measured.Incorporation of sulfonic acid groups was not identified.

Referenced Example 4

In dimethylsulfoxide 6.1 g of the hydrophilic prepolymer HP13 obtainedin Example 13 and 7.4 g of “SUMIKAEXCEL 7600P” were mixed and dissolvedat room temperature to obtain a 20 wt % solution. The solution waspoured into water to obtain a solid product. A blend composed of ahydrophilic prepolymer and a hydrophobic prepolymer was prepared. In 90g of 98% sulfuric acid 10 g of the obtained blend were dissolved. Afterthe resulting solution was stirred at room temperature for 24 hours, thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. The resulting solid productwas washed in water 5 times. The ion exchange-capacity of the obtainedpolymer was not able to be measured. In the H-NMR measurement, only thesignals derived on the hydrophobic prepolymer were observed. This resultindicates that, in the case of a blend that is not a block copolymer,the hydrophilic prepolymer becomes water-soluble after sulfonation, andis removed in the water-washing process.

TABLE 3 Proton conductivity for Example 12 and Example 13 (70° C.)Relative Proton conductivity (Scm⁻¹) Humidity RH % Example 12 Example 1390 2.62 × 10⁻¹ 2.07 × 10⁻¹ 50 2.70 × 10⁻² 2.03 × 10⁻² 30 6.55 × 10⁻³3.66 × 10⁻³

Example 15 Polymerization of Block Polymer PB15

In a flask, 28.7 g of bis(4-chlorophenyl)sulfone, 18.9 g of4,4′-biphenol, and 16.8 g of potassium carbonate were charged. After 160ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under a nitrogen gas atmosphere. Thetemperature was elevated to 175° C. while removing the generating watertogether with toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a polymer solution A. Separately, asolution was prepared by dissolving 73 g of “SUMIKAEXCEL 7600P” in 290ml of dimethylsulfoxide. This solution was added to the polymer solutionA, and the mixed solution was stirred at 170° C. for 1.5 hours. Thesolution was poured into a large amount of water to deposit a whitesolid product, which was then filtered off. The solid product was washedin hot water twice, and in methanol once to obtain a block polymer PB15.The solution viscosity η_(sp/c) of the obtained polymer was 0.71 dl/g.

Sulfonation of Block Polymer PB15 (SPB15 Synthesis)

20 g of the block polymer PB15 were dissolved in 200 ml of 98% sulfuricacid, and the resulting solution was stirred at room temperature for 24hours. The solution was poured into a large amount of water to deposit awhite solid product, which was then filtered off. Thus obtained solidwas washed in water 5 times to obtain a polymer SPB15. The ion-exchangecapacity of the polymer was 1.43 mmol/g. In the membrane prepared by themethod described below, a phase-separation structure was observed by TEMobservation. Hence, the polymer was identified to be a block copolymer.

Membrane Preparation and Peeling Properties Test

SPB15 was dissolved in N,N-dimethylacetoamide to obtain a 20 wt %solution. To the solution, monostearylphosphate triethanolamine salt wasadded in an amount of 0.5 wt % with respect to the polymer component toobtain a SPB15 solution. The solution was cast on a stainless steelplate (SUS304, 2 mm thick, 0.8S of surface finish), and dried with hotair at 130° C. for 1 hour. The resulting membrane was able to be peeledoff from the stainless steel plate easily.

Alkali Aqueous Solution Treatment and Acid Treatment

The resulting membrane was fixed on a stainless steel frame and heatedat 200° C. for 30 minutes. The membrane was detached from the frame,immersed in a 1 N NaOH aqueous solution at 25° C. for 2 hours,water-washed, and then immersed in a 1 N H₂SO₄ aqueous solution for 4hours. After water-washed twice, the membrane was fixed on a stainlesssteel frame and dried at 50° C. Thus obtained membrane had a thicknessof 35 μm and a proton conductivity of 8.0×10⁻³ S/cm at 60% RH. On theother hand, the membrane obtained after the heating at 200° C. for 30minutes had a proton conductivity of 2.3×10⁻³ S/cm under the samecondition.

Comparative Example 8

A membrane was prepared in the same manner as in Example 15, except thatmonostearylphosphate triethanolamine salt was not added. The membranefirmly adhered to the stainless steel plate. The membrane was tried tobe peeled off, but the membrane was broken off from the edge.

Example 16 Synthesis of Random Copolymer PR2

25.42 g of bis(4-fluorophenyl)sulfone, 15.93 g of bis(4-hydroxyphenyl)sulfone, and 6.77 g of 4,4′-biphenol were charged; after 170 ml ofdimethylsulfoxide and 50 ml of toluene were added, the reaction mixturewas heated and stirred under a nitrogen gas atmosphere. The temperaturewas elevated to 175° C. while removing the generating water togetherwith toluene, and then the reaction mixture was stirred at thattemperature for 16 hours to prepare a polymer solution. The solution waspoured into a large amount of water to deposit a white solid product,which was then filtered off. Thus obtained solid product was washed inhot water twice, and in methanol once, whereby a polymer PR2 wasobtained. The solution viscosity η_(sp/c) of the obtained polymer PR2was 0.53 dl/g.

Sulfonation of Random Copolymer PR2

In 200 ml of 98% sulfuric acid 20 g of the random copolymer PR2 weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid product was washed in water 5 times to obtain a polymerSPR2. The ion-exchange capacity of the polymer was 1.41 mmol/g. In themembrane prepared by the method described below, a phase-separationstructure was not observed, and the membrane was found to be uniform byTEM observation. Hence, the polymer was identified to be a randomcopolymer.

SPR2 was dissolved in N,N-dimethylacetoamide to prepare a 20 wt %solution. “SEPARL 365” (manufactured by Chukyo Yushi Co., Ltd.) wasadded to the solution in an amount of 0.3 wt % with respect to thepolymer component to prepare a SPR2 solution. The solution was then caston a stainless steel plate (SUS304, 2 mm thick, 0.8 of surface finish),and dried with hot air at 130° C. for 1 hour. The resulting membrane wasable to be peeled off from the stainless steel plate easily. Thethickness of the membrane was 38 μm.

Comparative Example 9

A membrane was prepared in the same manner as in Example 16, except that“SEPARL 365” was not added. The resulting membrane firmly adhered to thestainless steel plate. The membrane was tried to be peeled off, but themembrane was broken off from the edge. The thickness of the membrane was37 μm.

Example 17

In 100 ml of 98% sulfuric acid, 10 g of a commercially availablepoly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(available from Aldrich, 20,800 of weight average molecular weight,10,300 of number average molecular weight, and 322° C. of melting point)were dissolved. The resulting solution was stirred at room temperaturefor 45 hours. A polymer was deposited from the solution with water, andwas washed with a large amount of water until the pH of the washingwater became neutral. The resulting polymer was vacuum-dried at 60° C.to obtain a sulfonated polyetheretherketone. The ion-exchange capacityof the obtained polymer was 1.54 mmol/g.

The sulfonated polyetheretherketone was dissolved inN-methyl-2-pyrolidone to obtain a 20 wt % solution. “SEPARL 365” wasadded in an amount of 0.5 wt % with respect to the polymer component toprepare a solution. The solution was cast on a stainless steel plate(SUS304, 2 mm thick, 0.8 of surface finish), dried with hot air at 130°C. for 1 hour. The resulting membrane was able to be peeled off from thestainless steel plate easily. The thickness of the membrane was 30 μm.

Comparative Example 10

A membrane was prepared in the same manner as in Example 17, except that“SEPARL 365” was not added. The resulting membrane firmly adhered to thestainless steel plate. The membrane was tried to be peeled off, but themembrane was broken off from the edge. The thickness of the membrane was33 μm.

Example 18

13.41 g of 1,4,5,8-Naphthalenetetracarboxylic dianhydride, 17.62 g of anaromatic diamine having the following formula, 4.87 g of1,3-bis(4-aminophenoxy)benzene, 155 g of m-cresol, and 12 g oftriethylamine were stirred at 80° C. under nitrogen flow (30 mL/min)until a uniform solution was obtained. The temperature of the resultingsolution was elevated to 175° C. and the solution was stirred for 24hours to obtain a sulfonated polyimide solution. To 100 g of theobtained viscous solution (20.8 wt % of solid content, as calculated byassuming that the sulfonic acid groups were in a triethylamineamine saltform), “SEPARL 365” was added in an amount of 0.2 wt % with respect tothe polymer component to obtain a solution. The solution was cast on astainless steel plate (SUS304, 2 mm thick, 0.8 of surface finish), anddried with hot air at 130° C. for 1 hour. The resulting membrane wasable to be peeled off from the stainless steel plate easily. Thethickness of the membrane was 27 μm.

Comparative Example 11

A membrane was prepared in the same manner as in Example 18, except that“SEPARL 365” was not added. The resulting membrane firmly adhered to thestainless steel plate. The membrane was tried to be peeled off, but themembrane was broken off from the edge. The thickness of the membrane was27 μm.

Example 19 Polymerization of Polyethersulfone Block Copolymer PB19

In a four-necked flask equipped with a stirrer, a Dean-Stark trap, athermometer, and a nitrogen gas inlet, 10.71 g ofbis(4-fluorophenyl)sulfone, 27.5 g of3,3′-disulfo-4,4′-difluorodiphenylsulfone sodium salt, 18.9 g of4,4′-biphenol, and 17.5 g of potassium carbonate were charged; after 210ml of dimethylsulfoxide and 50 ml of toluene were added, the reactionmixture was heated and stirred under nitrogen flow. The temperature waselevated to 175° C. while removing the generating water together withtoluene, and then the reaction mixture was stirred at that temperaturefor 16 hours to prepare a hydrophilic prepolymer HP19 solution.Separately, 81.47 g of bis(4-fluorophenyl)sulfone, 78.99 g ofbis(4-hydroxyphenyl) sulfone, and 52 g of potassium carbonate werecharged. After 600 ml of dimethylsulfoxide and 50 ml of toluene wereadded, the reaction mixture was heated and stirred under nitrogen flow.The temperature was elevated to 175° C. while removing the generatingwater together with toluene, and then the reaction mixture was stirredat that temperature for 16 hours to prepare a hydrophobic prepolymerSP19 solution. The SP19 solution was added to the HP19 solution, and themixed solution was stirred at 170° C. for 1.5 hours. The solution waspoured into a large amount of water to deposit a white solid product,which was then filtered off. Thus obtained solid product was washed inhot water twice, and in methanol once to obtain a block copolymer PB19.The solution viscosity η_(sp/c) of the obtained polymer was 0.61 dl/g.The ion-exchange capacity was 0.59 mmol/g.

Sulfonation of Block Copolymer PB19 (SPB19 Synthesis)

In 200 ml of 98% sulfuric acid, 20 g of the block copolymer PB19 weredissolved, and the resulting solution was stirred at room temperaturefor 24 hours. The solution was poured into a large amount of water todeposit a white solid product, which was then filtered off. Thusobtained solid was washed in water 5 times to obtain a sulfonated blockcopolymer SPB19 composed of a hydrophilic segment having the randomcopolymer structure represented by the following formula (30) and ahydrophobic segment having the structural unit represented by thefollowing formula (31). The ion-exchange capacity of the obtainedpolymer was 1.46 mmol/g. If the polymer is a blend, the hydrophilicpolymer that is water-soluble is removed by water washing aftersulfonation and the ion-exchange capacity is largely lowered. However,the ion-exchange capacity of SPB19 gave almost the same value as thecalculated value, 1.49 mmol/g obtained by assuming that one sulfonicacid group was incorporated into each of the biphenol residue aromaticrings of PB19 before sulfonation. This indicates that SPB19 is not ablend, and that the hydrophilic segment and hydrophobic segment arebonded together. In addition, in the membrane prepared in the samemanner as described below, a phase-separation structure was observed byTEM observation. Hence, SPB19 was identified to be a block copolymer.The hydrophobic segment is not sulfonated under the foregoingsulfonation conditions, so that only the hydrophilic segment hassulfonic acid groups.

wherein, p and q, each represents a mole fraction of each structuralunit; and p=0.6 and q=0.4.

<Membrane Preparation>

SPB19 and “SEPARL 365” in an amount 0.5 wt % with respect to the polymercomponent were dissolved in N,N-dimethylacetoamide to prepare a 20 wt %solution. The solution was cast on a stainless steel plate (SUS304, 2 mmthick, 0.8 of surface finish), and dried with a hot air at 130° C. for 1hour. The membrane was able to be peeled off from the stainless steelplate easily. The thickness of the membrane was 24 μm.

Comparative Example 12

A membrane was prepared in the same manner as in Example 17, except that“SEPARL 365” was not added. The resulting membrane firmly adhered to thestainless steel plate. The membrane was tried to be peeled off, but themembrane was broken off from the edge. The thickness of the membrane was27 μm.

INDUSTRIAL APPLICABILITY

The present invention provides a sulfonated aromatic polymer electrolytethat is inexpensive and durable and keeps high proton conductivity, amembrane of the polymer electrolyte, and a use thereof.

Further, the present invention provides a polymer electrolytecomposition that does not lower largely proton conductivity at lowhumidity, a membrane that is composed of the polymer electrolytecomposition, and a fuel cell using them.

Still further, the present invention provides a method for producing asulfonated polyarylether block copolymer that is inexpensive and durableand keeps high proton conductivity, and a block copolymer obtained bythe method.

Still further, the present invention provides a method for producing apolymer electrolyte membrane, wherein a self-supporting membrane can beeasily peeled off from a support such as a metal belt, for example astainless steel belt, when a membrane composed of a polymer electrolytehaving strong acid groups or superstrong acid groups is produced.

1. A method for producing a polymer electrolyte membrane, comprisingproducing a polymer electrolyte membrane having a strong acid group or asuperstrong acid group by casting, wherein a polymer electrolytesolution containing from 0.0005 to 2 parts by weight of a phosphateester represented by the following formula (11) and/or a salt between anamine represented by the following formula (12) and a phosphate esterrepresented by the following formula (11) with respect to 100 parts byweight of a polymer electrolyte, is cast on a support, heated until asolvent of the solution is evaporated to form a self-supportingmembrane, and the self-supporting membrane is peeled off from thesupport;

wherein R¹ is a hydrogen atom, an alkyl group having 6 to 18 carbonatoms, or a group represented by the following formula (13); and R² isan alkyl group having 6 to 18 carbon atoms or a group represented by thefollowing formula (13),[Formula 12]R³—(OC₂H₄)_(m)—  (13) wherein R³ is an alkyl group having 5 to 18 carbonatoms; and m is an integer of from 2 to 30,

wherein R₄ to R₆ each are a hydrogen atom, a hydroxyethyl group, or analkyl group having 1 to 12 carbon atoms.
 2. The method for producing apolymer electrolyte membrane according to claim 1, wherein theself-supporting membrane is further treated with an acid aqueoussolution.
 3. The method for producing a polymer electrolyte membraneaccording to claim 1, wherein the self-supporting membrane is furthertreated with an alkali aqueous solution, and then with an acid aqueoussolution.
 4. The method for producing a polymer electrolyte membraneaccording to claim 1, wherein the polymer electrolyte is a hydrocarbonpolymer electrolyte.
 5. The method for producing a polymer electrolytemembrane according to claim 4, wherein the hydrocarbon polymerelectrolyte is an aromatic polymer electrolyte having an aromatic ringin its main chain.
 6. The method for producing a polymer electrolytemembrane according to claim 5, wherein the aromatic polymer electrolyteis an aromatic polyether having a structural unit represented by thefollowing formula (14):[Formula 14]—Ar¹—Z¹—Ar²—Z¹  (14) wherein Ar¹ and Ar² each are an aromatic group; andZ¹ is an oxygen atom.
 7. The method for producing a polymer electrolytemembrane according to claim 1, wherein the polymer electrolyte is acomposition of a polymer electrolyte having sulfonic acid groups and apolymer having no sulfonic acid group.
 8. The method for producing apolymer electrolyte membrane according to claim 1, wherein the polymerelectrolyte is a block copolymer containing a hydrophilic segment havingsulfonic acid groups and a hydrophobic segment having no sulfonic acidgroup.
 9. A polymer electrolyte membrane produced by the method forproducing a polymer electrolyte membrane according to claim 1.